You could pump both after he feeds. Did you pump after feeds the whole time in hospital? I am sure it will be fine - as you can see from my post I had a huge supply.
As everyone else said wet diapers, content baby are the best ways to tell. We bought a baby scale to have at home to make sure he was gaining. The nurse told me not to get one and I was being neurotic but I only weighed him every second day (not after every feed like in hospital!) this way i knew he was getting bigger and made me feel more secure about what I was doing. Graco makes one for about $60.
Good luck- you will be so happy you are doing even though I know how hard it can be especially with the NICU expereience thrown into the mix.
I have heard preemies can be more difficult becasue their sleep cycles are different etc but the bf'ing and co-sleeping brought me and baby into sync and helped us bond after being semi-seperated for so long. The breast fixed everything, every cry whimper or squeal. After about the first week, Dean didn't cry and was the easiest baby I had ever heard of and still is! I know your experience will be just as wonderful.