Four great elements : what is the other 2 elements?

As disciples of the Buddha it is good to view the four physical elements (dhatu) of nature: Earth, Water, Fire and Wind (pathavi, apo, tejo, and vayo) as Elements of solidity, fluidity, heat and motion.
(1) Earth element (pathavi-dhatu) has the property of hardness, strength, thickness, immobility, security and supporting. - they represent the solidity property of forms. (2) Water element (apo-dhatu) has the property of oozing, humidity, fluidity, trickling, permeation and cohesion. - they represent the fluidity or cohesiveness property of forms. (3) Fire element (tejo-dhatu) has the property of heating, warmth, consuming and grasping. - they represent the heat or cold temperature property of forms. (4) Air element (vayo-dhatu) has the property of motion, supporting, coldness, ingress and egress, easy movement and grasping. - they represent the distended-ness property of forms.

These four elements are there in any form, including our body. In Girimananda Sutta (AN 10.60) The Buddha tells Ananda:

“Herein, Ananda, a monk contemplates this body upward from the soles of the feet, downward from the top of the hair, enclosed in skin, as being full of many impurities. In this body there are hairs on the head, body hairs, nails, teeth, skin, flesh, sinews, bones, marrow, kidneys, heart, liver, pleura, spleen, lungs, intestines, intestinal tract, stomach, feces, bile, phlegm, pus, blood, sweat, fat, tears, grease, saliva, nasal mucous, synovium (oil lubricating the joints), and urine. Thus he dwells contemplating foulness in this body. This, Ananda, is called contemplation of foulness.”

The last two Dhatu – Akasa & Viññaña can be seen in this way. Akasa is of two types. The one in Arupa Jhana is the sphere of infinite space (Akasanañcayatana). This is attained by transcending any cognition of rupa, by abandoning the metal image that was previously the object of concentration, and seeing that space is infinite. The Akasa Dhatu is more limited. It is the space element, belongs to derived materiality. This element has the characteristic of delimiting matter or form, and indicates the boundaries of matter. Akasa is the container of matter or form being untouched by the 4 great elements, and in holes and openings. It is for Akasa element that one can say 'this is above. below, around that'.

Viññāna is the 5th in the sixfold division of elements dhātu. It is the mind-consciousness-element. Like the other 5 Dhatu, this too is impersonal. Consciousness, when you're aware of it inwardly, arises and passes away by its very own nature. There's no real essence to it. Viññāna as dhatu or elements, are divided into 7 classes:

1. cakkhu-vinnana-dhatu = cakkhu-vinnanadvi. They depend on cakkhu-vatthu for their arising. (eyes)

2. sota-vinnana-dhatu = sota-vinnanadvi. They depend on sota-vatthu for their arising. (ears)

3. ghana-vinnana-dhatu = ghana-vinnanadvi. They depend on ghana-vatthu for their arising. (nose)

4. jivha-vinnana-dhatu = jivha-vinnanadvi. They depend on jivha-vatthu for their arising. (tongue)

5. kaya-vinnana-dhatu = kaya-vinnanadvi. They depend on kaya-vatthu for their arising. (body)

6. mano-dhatu = panca-dvaravajjana and sampaticchanadvi. They depend on hadaya-vatthu for their arising. (mind)

Both @user1387280 and @ChrisW were not sure when it came to this OP. There is nothing wrong in this. Even I do not know much about the last two Dhatu – Akasa & Viññaña. There is another deeper level of understanding to Dhamma beyond our empirical surface experience of what we read or hear of Dhamma. That is why Venerable Ajahn Chah once said, ‘Not Sure!’ is the standard of the Noble Ones. Only those who really do not know what practice is would say “we know”.

I'm not sure this is useful to you but here is some further reading.

As written in user1387280's answer, various "6 elements" (including those you mentioned i.e. earth, wind, fire, and water, plus space and consciousness) are listed in the Bahu Dhātuka Sutta (MN 115).

Footnote 21 to the phrase "eighteen elements" says,

1 Def at Vbh §§183-184/87-90; expl in detail at Vism 15.17-43/484-490. See Intro (1) above

There's also a description of each of the six elements in the Dhatu-vibhanga Sutta (MN 140).

I'm not sure they do "connect" with each other (maybe what makes them "elements" is that one can exist without the other). Still insofar as they are elements of a person they are a put-together assemblage. The teaching that applies to all of them is that they're all not mine, not me, not myself.

Nevertheless, Chapter 15 of the Visuddhimagga discusses "elements" and says, for example,

30. The earth, fire, and air elements are the tangible-data element; the water element and the space element are the mental-data element only; “consciousness element” is a term summarizing the seven consciousness elements beginning with eye-consciousness

Footnote 32 on page 443 explains why water is 'mental': it's because 'water' is the mind's perceiving an absence of 'fire'.

I assume (speculate) the four great elements was an understanding before the Buddha found in many cultures (see Wikipedia: classical elements). In ancient times, different cultures or philosophies would add onto the four great elements other elements they regarded to be important.

The Chinese had a somewhat different series of elements, namely Fire, Earth, Metal (literally gold), Water and Wood, which were understood as different types of energy in a state of constant interaction and flux with one another, rather than the Western notion of different kinds of material.

In respect to the four great elements, the Buddha used this in his definition of the physical body for the purposes of Dependent Origination, i.e., to show how ignorance-created-formations (sankhara) condition stress & agitation in the physical body (rupa):

What is mentality-materiality (nama-rupa)? Feeling, perception, volition, contact and attention — these are called mentality. The four great elements and the material form derived from the four great elements — these are called materiality.

Sammaditthi Sutta

In respect to the six elements, the Buddha was concerned with those non-personal (anatta; sunnata) elements (dhatu) that could be directly discerned/experienced by the mind in their most natural state. He emphasized earth (solidity), wind (breath; wind), fire (heat; including body heat), water ( moisture, softness), space (internal & external) and primal consciousness (awareness).

Refer to: Mahārāhulovāda Sutta & Dhātuvibhaṅga Sutta

Bhikkhu, this person consists of six elements. There are the earth element, the water element, the fire element, the air element, the space element and the consciousness element.

What, bhikkhu, is the earth element? The earth element may be either internal or external. What is the internal earth element? Whatever internally, belonging to oneself, is solid, solidified, and clung-to, that is, head-hairs, body-hairs, nails, teeth, skin, flesh, sinews, bones, bone-marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, contents of the stomach, feces, or whatever else internally, belonging to oneself, is solid, solidified, and clung-to: this is called the internal earth element. Now both the internal earth element and the external earth element are simply earth element. And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’ When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the earth element and makes the mind dispassionate towards the earth element.

What, bhikkhu, is the water element? The water element may be either internal or external. What is the internal water element? Whatever internally, belonging to oneself, is water, watery, and clung-to, that is, bile, phlegm, pus, blood, sweat, fat, tears, grease, spittle, snot, oil-of-the-joints, urine, or whatever else internally, belonging to oneself, is water, watery, and clung-to: this is called the internal water element. Now both the internal water element and the external water element are simply water element. And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’ When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the water element and makes the mind dispassionate towards the water element.

What, bhikkhu, is the fire element? The fire element may be either internal or external. What is the internal fire element? Whatever internally, belonging to oneself, is fire, fiery, and clung-to, that is, that by which one is warmed, ages, and is consumed, and that by which what is eaten, drunk, consumed, and tasted gets completely digested, or whatever else internally, belonging to oneself, is fire, fiery, and clung-to: this is called the internal fire element. Now both the internal fire element and the external fire element are simply fire element. And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’ When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the fire element and makes the mind dispassionate towards the fire element.

What, bhikkhu, is the air element? The air element may be either internal or external. What is the internal air element? Whatever internally, belonging to oneself, is air, airy, and clung-to, that is, up-going winds, down-going winds, winds in the belly, winds in the bowels, winds that course through the limbs, in-breath and out-breath, or whatever else internally, belonging to oneself, is air, airy, and clung-to: this is called the internal air element. Now both the internal air element and the external air element are simply air element. And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’ When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the air element and makes the mind dispassionate towards the air element.

What, bhikkhu, is the space element? The space element may be either internal or external. What is the internal space element? Whatever internally, belonging to oneself, is space, spatial, and clung-to, that is, the holes of the ears, the nostrils, the door of the mouth, and that aperture whereby what is eaten, drunk, consumed, and tasted gets swallowed, and where it collects, and whereby it is excreted from below, or whatever else internally, belonging to oneself, is space, spatial, and clung-to: this is called the internal space element. Now both the internal space element and the external space element are simply space element. And that should be seen as it actually is with proper wisdom thus: ‘This is not mine, this I am not, this is not my self.’ When one sees it thus as it actually is with proper wisdom, one becomes disenchanted with the space element and makes the mind dispassionate towards the space element.

Then there remains only consciousness, purified and bright. What does one cognize with that consciousness? One cognizes: ‘This is pleasant’; one cognizes: ‘This is painful’; one cognizes: ‘This is neither-painful-nor-pleasant.’...

Dhātuvibhaṅga Sutta

Four great elements : what is the other 2 elements?

In the Nikaya Texts, there is a term called 'Cattari Mahabhutani'(four Mahabhuta). These Mahabhutani consitute Rupa. (See Sammaditthi Sutta).

A being (human being is an instance) is defined in terms of six Dhatus. It is Cattari Mahabhutani and Akasa and Vinnana Dhatus (Caddhaturo Ayam Puriso) (See for example Bhahudhatukasuttam.) However, there are no meaningful translations of these Suttas in any language. These Suttas could be understood only by Lord Buddha's direct disciples, who listened to Lord Buddha in person. This non-understanding is the reason for the existence numerous Buddhist traditions and sects.