I breastfed twins until they were 1, exclusively breastfed (apart from solids too) until about 7.5 months at which point we tried to introduce ebm in a bottle.
They were my first babies (you have my upmost respect for wanting to do this with 3 children already) so I didn't know every different, took a while for them to get a good latch (they were premature so we used nipple shields for 5 or 6 weeks) but I had quite alot of milk and they gained weight well once they were out of special care. I had masses of help from the midwives on the unit and the community midwife who came to visit when we were allowed home.
I never got the hang of tandem feeding, I would feed one whilst the other was in a bouncy chair that I rocked with my foot and sang! Seemed to work. In the early days it was time consuming but very quickly they got to be quite efficient feeders. They would both be done in 20 mins.
I also "dedicated" one boob to each baby - twin 1 was always the right, twin 2 was always the left. Worked for me (and I didn't end up with a lopsided chest!). I think that helped with them establishing the latch, always being in the same position.
I also used to feed them both - always (until about 7 months when DS started sleeping through). In the night, if one woke for a feed, I woke the other up too. At least that way you were guaranteed (well kind of) a bit of a stretch for sleep. I also had help in the night - I would be feeding 1, but H would do the nappy change / settling for Number 2. Meant that although we were both up each time they needed a feed, it was only for about 20-30mins.
Good luck!