I think it would depend how you use your cars. For example, if one car is only ever used for driving one of you to work, then I'd just get seats for that one. If you are likely to use both cars for family trips, then it would probably make life easier if you are able to get seats for both cars.

Personally if I already had a seat in each car for one baby, I'd be likely to get another 2 seats if I had 2, so both cars were ready to use (assuming finances allowed, of course!).