In a comment, Cougar mentioned a family where one person speaks English and the other Polish.
There are a few kinds of conversations mutually bilingual people will have. You’ll have the ones where people switch between languages whenever they feel like it — by word or by sentence. You’ll have the ones where each person just speaks in the language they’re more comfortable in. (This is, for instance, how I correspond with a friend — in speech it varies, though mostly is English, but he almost only writes me in French, and I wouldn’t dare try mine. It’s also quite common in romantic relationships and between generations in families.)
My favourite kind is one that is almost entirely limited to service relationships — an employee at a coffee shop, say. I go and order in French, and they can (usually) tell that I am an anglophone (often because I start by saying ‘uh’ instead of ‘euh’). So they respond to me in English, trying to be helpful. I can tell they’re a francophone, so I respond to *them* in French. With conversation limited to talk about coffee, it’s not really a big deal (anymore, though sometimes I forget that), but you still end up with both people speaking in their second language and having comprehension just that much harder, to be helpful.