It's case by case. There's not really a universal yes or no that properly answers your question. Some instances actually have slots for only 1 tank (with 2 healers and 5 DPS). As for the ones such as you describe with 2 tanks, those two slots are there for a reason. However, individual preferences will still lead to variance at times; there's more than one way to do things. Also, some of the older instances used to greatly benefit from dividing the tanking but have become easier over time as people's gear improves beyond what it was at the duty's debut. And sometimes the situation is more complicated than just who's tanking; sometimes adds need to be taken to specific areas and so forth.

So ... we could offer more guidance if you say specifically which duty you're talking about. But in general, safe bet would be to just ask the party what they want.