This is an area where the choice is often idiomatic. For example, in NZ English we would say "two and two is four", which would suggest that we would use your option number 1. In fact, of the two options you offer, the one you would hear most often in NZ is number 2. Perhaps because the insertion of the sibilant helps the words flow together. This would seem to me to be analogous to "mon amie" in French - not following an expected grammatical structure, but with a clear reason for the deviation.