I also have an Altima. I don't see an issue fitting two car seats in it. DD is only 15 months, and I will be keeping her rear facing until 2 at least.
The issue is that is works fine for ME!!!! I am 5'2" so I don't worry about space too much. But....my husband is 6'2", and there is no way that he'll be able to drive my car.
He drives an Accord. I worry that is also going to be too tight for him because of how far back he has his driver seat. We are ready for a new car anyway, so we are trying to figure out what to buy before the new baby arrives. And yes, we will be bringing both car seats with us to test that they actually fit in the cat before we buy anything!