I breastfed my twins (DD2 and DS) for 16 months. I also had a lot of trouble breastfeeding my first Dd, and wound up pumping for her for a long time and then losing my supply and going to formula. With the twins, it was so clear to me that full-time formula was unaffordable for us, so I knew from the beginning breastfeeding was the only way to go, and we MADE it work. It was not without its challenges, and both twins had small amounts of supplementation in the early months, but we did it and I feel very proud of that.
DD2 had some supplements in the first two weeks because she was small, very jaundiced, and sluggish with it, and wasn't transferring milk because of her sluggishness and a weak suck. DS got some supplementation at around 2 to 3 months old, because I was very sick (I have a serious chronic illness) and my supply was dipping.
If supplementation does become necessary, an SNS or a Lact-aid can allow you to supplement at the breast, so that you're not starting a downward supply towards early weaning. For both my kids, the SNS meant that the supplements were a very short-term thing, and we returned to exclusive breastfeeding very quickly.
My best advice for twins is that it is going to be a full-time job. You will want to start lining up help so that when they arrive, nursing them is your only responsibility for a few weeks. That's tough--- I had a 2 year old in the house when my twins came, and once my "help" went home after a week, I was basically on my own, and it was very difficult. If I'd had one of these spouses who refuses to get up at night with a baby, or hadn't had my mom nearby and a friend who would come over to take DD1 out for a few hours, I would have gone mad.
In the early weeks, I fed each twin separately, because they each needed so much attention staying latched, and because DD2 needed help staying awake and focused on feeding. Once we learned to nurse both at the same time, that did save a lot of time, although there were many phases where we want back to separate feeding for many reasons.
Honestly, the breastfeeding wasn't the hardest part of twins. The never-ending diapers drove me much crazier, as did DS's refusal to nap.