I BF both of mine up till 7 months old. Tom is still BFing but only cos I can't get the stubborn lil bugger to take a bottle but that's another story.
Once we got into the hang of things it's been absolutely fine. I use one of these cushions:
https://i13.photobucket.com/albums/a283/HelenandBill/GetImageaspxfilenametw0008.jpg
which is really solid and comfortable and feed them rugby ball style.
For the first couple of weeks I fed them one at a time till I got the hang of breast feeding and felt confident enough to try like this. Feeding them both at once saves loads of time and stops you having to listen to one cry while you finish the first one.
It can be a bit of a hassle at first trying to get them to feed at the same time and keep them in synch. Work on the assumption that if one wakes for a feed that you will wake the other one as well. It can be a bit tough mentally to wake a sleeping baby but it's far better to do that than to settle down to sleep yourself only to be woken half an hour later. I had two little wheel things to help me keep track of who had fed when or I wrote it on scraps of paper and post-it notes!
Within a few weeks we were pretty much all in synch and their feeding settled down to a more regular pattern.
Each baby had their own boob (again this didn't happen straight away, I used to swap around) because my MW said that a baby will stimulate the breast to get it to produce the milk they need. I figured if they had their own boob then the milk that that boob produced would be right for them IYKWIM. Plus Tom was better with the slower boob and Charlotte with the faster one.
From about 6 weeks, I expressed milk in the morning that they had in a bottle for their late feed because I'd read somewhere that this would help them to sleep through. Dunno how true that is, but mine both slept through from 10 weeks old.
The other thing to be prepared to do is EAT! I wasn't big to begin with but I'm about a stone lighter than I was before I was PG and it's all down to the BFing (that and the walking with the buggy and manic running around you do as a twin mummy
). I've been told by my HV that I'm buring 1000 calories a day BFing two. Get the high calorie snacks in!
I started with pretty low expectations of how long it would be possible for and I've had my low points usually around growth spurts but we've got over those. I thought if I could get to 6 months that would be a miracle, yet here we are!
It is possible. You can do it and boy can you feel smug about it!
People always so have difficult it is to BF twins but never mention how difficult it would be to sterilize, make formula, clean bottles for two babies. 
I'd make my formula up in the morning and make two or three bottles for over night 
