In British English, we are completely flexible about the issue of singular and plural when it comes to singular words which describe a group (or in this case, a pair) of people. If we are considering them as individuals, then we can use a plural verb and plural pronouns (they,their, them), or if it seems more appropriate to the context, we can also treat 'couple' as a singular noun.
In this case, I would say 'are supposed to be'.
American English tends to be more rigid in this regard. Nouns such as family, team and so on are always singular, and I suspect that couple would be too.
Let's see if any American English speakers can offer any insights on this question.