"Is" is the correct verb here actually because "one" is the subject. "Marriages" is inside the prepositional phrase "in two marriages" so it is not the subject. That being said, this is a grammar rule we often ignore, especially in speech or conversation :)

I don’t think so, because you’re talking about the marriages, for me sounds better ‘are’

I agree with Jessica. The subject is "one" which is singular. The verb should be "is". "One" is modified by a preposition phrase, "in two marriages". If this is a question on a grammar test, I would use "is". However, when speaking many people will say "are" because they just said "marriages" which is plural and everyone is starting to forget about "one"; and also because one in two is fifty percent and that is a large number of marriages.

Hey! "Are" the correct word in this instance because the noun marriages is plural. If the noun is not plural then the correct word is "is" for example "My marriage is falling apart". Hope this helps! :)

Is corresponds to the subject one..... Take for example... one of them is/are coming?