I wish I had seen your question sooner. I am a mother of twins who was able to breastfeed them successfully throughout their infancy. Those little girls grew up to have babies of their own which they also breastfed.:)
I had advice from another mom of twins from my local La Leche League which made all the difference. I know this might be coming too late now, but just in case it is not --
I drank 15 glasses of water a day. I increased my milk supply by feeding in short, frequent bursts, and over a couple of days the milk supply grew substantially.
That's pretty much it. Sounds really simple but if I hadn't been told to try this I would never have been able to feed them.
If you have switched to bottle-feeding, please don't feel bad! It's not easy nursing twins, and without some help and information, it's impossible. My first baby was bottle-fed, and in that case I was disappointed and felt bad about myself because I had wanted so badly to breastfeed and circumstances prevented it.
But that baby is now a full-grown man who is healthy and strong, and 6'2" tall.:) And if you are bottle-feeding, there are advantages to it. Don't feel guilty about it! Your children can grow and thrive, and life can be good.:)
Good luck either way:)