The fact that you already nursed a baby puts you in a good situation, because you will have the ability to make even more milk with your 2nd (and 3rd) babies. An LC told me that, and it really helped ease my mind.
Whether you need formula really has to do with your supply in the beginning and, if you have preemies who can't nurse yet, how much you can pump. I only had to rely on formula because I couldn't pump enough, but by working hard at my supply I was able to transition them off of formula and EBF. That is very possible, but it is not easy.
I'd speak with an LC at the hospital when you check in but before the birth, to arrange for a pump to be brought to your room within a couple hours of delivery, regardless of how things work out. I had to have a c-section and my babies were taken to the NICU, so while I was stuck in my room by myself I was able to pump, eat, drink, and rest all the things that can really help your milk come in. Of course, hopefully you'll have a vaginal birth, and have your full term babies in your room, but the pump is still going to be really good to start early on.
The other thing to consider is bring a wrap to the hospital with you. I should have bought one before my twins were born, but I didn't realize how useful it would be for newborn twins! I didn't get my Moby until a few months later. You can do a double hold with them in there (in an upright hold with their faces against your upper chest), kind of like the first photos on this page:
http://babywearingtwins.wordpress.com/2010/03/04/gorgeous-guest-stars-zina
Skin to skin contact is a
huge factor in having milk come in and building a strong supply, and if you can strip them down to their diapers, take off your top, and keep them in a wrap for awhile, it will keep them warm against your skin and snug and keep you covered. Much easier than juggling two near-naked newborns in a hospital bed and trying to get them next to your skin with a hospital gown! (Actually, bringing button-up pajamas would be really useful for easy skin to skin access.) Don't know if you want to learn to do a twin hold with a wrap so soon after giving birth (takes some practice to do well), or if you have an experienced doula who could help you with it, but think about it as an option. If they are in the NICU you won't need it (the monitor wires won't work well with the wrap) but you'll absolutely want to do the skin to skin kangaroo care in there, too. I always wore button up shirts tot he NICU so I could unbutton and put the babies against my skin, one at a time. I just wrapped my shirt over them.
As for when else you'd have to supplement, besides a preemie situation there is the possibility of your milk coming in late, the babies having jaundice, being very underweight and needing some supplementation (like you adding formula to pumped breastmilk to increase the calories), or for various reasons you guys have a rough start and they lose a certain percentage of body weight in the days following their births (13% I think? something like that). The good news is that none of that means it has to be a long-term situation. The bad news is that you really can't plan or prevent any of that from happening.
If it does, you'll have to figure out how to roll with it for the time being, and without beating yourself up. One thing that really helped me was what our neonatologist said--"just think of formula as the medication they need right now." Perfect metaphor. It's a temporary thing they need and is only a part of the picture (you're doing a lot of other things for their health, too). They won't be on it forever but it's sure wonderful that it is there for their moment of need.
