Are there any good examples in earth history of two religions co-existing?

Another example in China are Confucianism (formally treated as religion), Taoism and Buddhism, and in particular, with regard to your question, the two last ones.

Many people have good examples of the Mongols, Japan, and other Asian countries, but in the western world other examples come to mind. West Africa is a good example. In Western Africa, many rulers and most merchants converted to Islam (e.g. the Dioula or Dyula), but the general population was still polytheistic. And everyone should note that The Islamic world was always tolerant of other religions, not attacking, allowing them to live peacefully though they did tax them more heavily. Until about the crusades, when they became less... happy with Christians; even then Judaism, Christianity, and many polytheistic religions still had freedom until in most Islamic empires until about the rise of religious revivalism in the 20th century.

• Mongols and any Christian territory ruled by them - such as Russia. While there was, of course, conflict (it was, after all, a war of conquest), it was NOT in any way, shape or form of a religious nature, but pure geopolitics. Mongols were explicitly, by design, very religiously tolerant and enforced that.

• Another example would be Armenia (which existed as a Christian nation since 301AD till Sassanid conquest; and was predominantly Zoroastrian - which is also pretty monotheistic for my taste - for a while before then); and any of the numerous pagans both in and around Armenia.

However, examples before 1700 are definitely NOT numerous, for a variety of reasons:

1. There were not many strictly monotheistic faiths in general before 1700 which lasted (3 main Abrahamic ones, Zoroastrians, Sikhism (possibly, not too familiar)).

   Worship of Aten in Egypt only lasted for ~20 years so doesn't count. Ba'hai are post 1700. Sabians can probably be considered monotheistic, but as far as I know never held any territorial soveregnity to be in a position to engage (ore refrain from) conflict with polytheistic religions.

2. Of the 3 Abrahamic ones, Judaism cannot be counted as fitting the question, by definition, because it is NOT expansionist in religious terms (Judaism prohibits both prozelatyzing and forced conversion).

3. Islam is explicitly hostile to polytheism.

   So is state-merged, post-Nicean post-Constantine Christianity.

My general impression is that in most great empires of the past, where the population was mixed, most of the times religions coexisted peacefully, like in Persian empire, for example. Some other examples are:

1. In Khazar khaghanate, Judaism, Islam and Christianity apparently peacefully coexisted with paganism and with each other.

2. In the Roman republic (and later Roman empire) all religions peacefully coexisted except Christianity in some periods when Christians and sometimes Jews were persecuted.

3. In Polish-Lituanian Commonwealth (Rzeczpospolita Obojga Narodów) several religions coexisted in relative peace (Catholic, Orthodox, various protestants, Muslims and Jews) until the 17 century wars between the Orthodox and the rest. Before the Commonwealth, Christian Orthodox peacefully coexisted with the pagans in the Grand Duchy of Lithuania. (This Great Duchy was the last state in Europe ruled by pagan princes).

4. Mongol empire is also a good example. I think it is generally accepted that Mongols were very tolerant to all religions. Some of them quickly converted to Christianity others to Islam and other religions, others remained pagan, but no problems related to religions seem to be recorded in Mongol empire.

EDIT (after reading the comments). It seems that religious prosecution mostly happened when the rulers were of monotheistic proselyting religions (like Islam and Christianity). Herodotus describes all the world known to him, and I don't remember him ever mentioning any religious persecution. However I am not sufficiently familiar with history of India and China to insist on this generalization.

An exact example which adheres to your definition might be hard to find, but I got one which is quite close.

In Japan, both Buddhism and Shintoism were practiced in parallel.

Buddhism came to Japan in the 6th century. Buddhism focuses on the worship of a single entity, but calling it monotheistic might be kind of a stretch considering that Buddha never claimed divinity (at least not in the sense God does according to the Abrahamic religions). It is not centralized under one leader atop an organized hierarchy (like, for example, Catholicism), but encourages practitioners to follow masters they choose for themselves. It is definitely not a violent religion, but still slightly expansionist because practitioners are encouraged to teach others. This is all grossly oversimplified. For more information, I recommend Buddhism Stackexchange.

The roots of Shinto can be traced back to various nature cults which existed far before written history and is as polytheistic as a religion can be. Shinto worships a countless number of Kami, which could be translated as "gods" or "spirits". The majority of kami are only worshiped regionally and there are many independent Shinto shrines with individual traditions and rites dedicated to one or more Kami. So it could be called "tribal" if you want to use that word.

With the exception of a brief episode in the 19th century (the Meiji restoration) both religions coexisted peacefully for centuries and still do until the present day. In current day Japan, nearly 80% of the Japanese people regularly practice Shintoism and around 34% self-identify as Buddhists. Yes, you calculated correctly, that's more than 100%. Many Japanese practice both religions simultaneously.

The two religions can coexist quite well because their believes and value systems do not contradict each other too much, neither claims to be infallible and neither encourages violence against non-believers.