I have been doing sleep training for my twins and have a question!!?

What happened was that when they were first born if one woke up we woke the other up. We did this thinking it would help us get a little sleep. Unfortunately all it did was train them to wake up. So after talking to my doc and friends we decided to try sleep training and letting them cry it out. My question is if they sleep 8 hours should I feed them? The doc thinks they should sleep 10-12, but if they wake after 8 aren't they legitimately hungry? I know if they wake up after 4 or 5 hours I am not supposed to feed them, but how many hours are too many hours? Thanks Mommies ( and Daddies!) Oh and they are 9 months old and very healthy happy eaters!