How does the Trinitarian defend this argument from a JW?

Open the Bible up to Matthew 22:30, and show him that in heaven there is no marriage. Therefore the eternal oneness between the Father and Son is entirely different than the "one flesh" described in the institution of marriage.

For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven. (Matt 22:30)

More importantly make sure you go with the permission of the Holy Spirit, because it is not by logic or eloquence that the heart is reached, but by the Spirit of God.

Finally, the mystery of redeeming love is something that angels desire to look into (1 Peter 1:10-12). None but God is capable of the the ultimate demonstration of self-sacrificing love at the cross. Only He who knew the height and depth of the love of God could make it known.

Husband and wife become "one" only in the same way light, heat and other things come together to constitute the one flame, yet are distinct. The husband and the wife are still distinct. Just as the Sender and the Sent are distinct from one another, by definition.

Analogously, the Father, Son and Holy Ghost are most certainly distinct in Scripture (e.g. one sends the other; submits their human will to the other, in the case of Jesus Christ; one recieves revelation or truth from one and conveys it to us, etc.). Yet if there is only one God (inarguably the teaching of the Old and New Testaments), then quite clearly these Persons (for they will and can be sinned against unto damnation, and speak messages, and make commands, and know all things, etc.) must somehow—God being ineffable—be one and the same God.

It is a logical fallacy (and thus an insufferable falsehood) to conflate two things in union with one another: union implies distinction in the first place (else what two things are in union?)

Unitarians who claim 'echad' (one in Hebrew) in fact means 'Unitarian' or 'Unipersonal' rob it of its clear context: a counter to the idea of polytheism (or, many-God-ism).

There are a few fundamental meanings for the word 'one:' one as in family (kinship, familial relationship); one in purpose, or agreement; or one as in an abacus (one counter, not two counters, etc.).

Which of these do they say God meant He is "one"? Clearly in the context it was said, it was a repudiation of idolatry, or polythiesm, or henotheism (YHVH, but others on the side, too).

We see Trinitarian formulae all over Scripture, e.g:

Matthew 28:19 (DRB)

Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.

1 Peter 1:2 (DRB)

According to the foreknowledge of God the Father, unto the sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you and peace be multiplied.

They make God a ventriloquist, claiming Jesus was not talking to His Father, but Himself on the Cross, or saying He was one with Himself, instead of with the Father. Saying He came forth from Himself and into the world, instead of came forth from God, and thus is God. That He has Himself for His own Father, instead of He has God for His Father.

It is absolutely ludicrous, and, as said, a fallacy, to claim that the husband and wife are literally one person.

The Father which is the "Holy Spirit" for God is Holy and a Spirit, dwels IN the Son Jesus!!! Ephesians 4:4 there is one spirit and one body!!! One faith, one Lord one baptism, of the Father, the Holy Ghost!!! Acts 2:38. The godhead or godhood, singular deity, the Holy Ghost, the Father, dwels in the Son!!! There are just 2, "two", the Father which is invisible and cannot be seen dwels in the Son. When you see God, you see Jesus. Not because Jesus is God, but because he is the image of the invisible God, Colossians 1:15, Luke 3:22 tells us God was in the world when he manifested in his Son Jesus!!! There is only one on the throne and that is Christ Jesus. Matthew 28:19 baptize in Father, Son, Holy Ghost. Well, read the book of acts, and count how many were baptized in titles. No one ever baptized anyone in those titles, for there is no remission/ pardon of sin in any other name than Jesus Christ!! You repent be baptized in Jesus name for remission of sin and get hands laid on and receive the Holy Ghost. Acts 2:38, acts 19 with speaking in tongues as evidence, Amen!! Then you will fulfill John 3:5 born of water and of the Spirit!!!

Another option would be to explain that the 'oneness' of husband and wife IS a perfect example of Trinity.

Both are equal in their humanity. Both have different roles IN the marriage. Certainly, a single man and a single woman do not have those same roles when they are not married. Yet, they are one unit - which is the meaning of the word "one" in the Hebrew. It is also used of a clump of grapes, i.e. one clump.

As such, though we may see 2 persons living in their separate bodies, they are in fact ONE family, one entity as it were. When they produce an offspring, the child is also part of the ONE family/ONE entity. The child is also equally human in nature, yet possesses a different role than the other 2.

Ta da. We have a trinity.

Furthermore, even in the tragedy of parental separation or divorce, GOD still sees that family as one unit, one entity. He is the one that says except for cases of infidelity a husband and wife are one flesh. Though our human eyes see a separation of the one, God does not.