hi, just wanted to tell you it sounds like you are doing everything as perfect as can be. just relax, i know its hard though. i have formula excl fed my first, and exclu breastfed the second. i can tell you the biggest difference is you dont know, or need to know the actual amount of ounces. stop worrying about that, as long as you have wet diapers, no soft spots, and happy babies, you are doing well.
do not think you cant breastfeed both, the percentage of women who actually dont produce enough milk for their babies is much smaller than the amount of those who say they didnt. although twins i harder to breastfeed, dont think you arent making enough. at 3 months, your breasts are adjusting and no longer feel full, making some women think they are drying up which isnt the case. dont worry about how long they feed, babies are all different. my sisters baby fed for 45 min straight at that age. my daughter fed about 5 minutes, 10 at the most. some babies are snackers, some are more efficient, some like to take their time. you cant compare as they are all different. at 3 months, i was feeding every two hours still, which i relate to the fact she ate only 5 minutes. 2-3 hours is common for the entire first year for some babies nursing though.
i am telling you, as long as you dont get in the cycle of thinking- thinking your not making enough, give some formula, now your body doesnt realize it needs to make more, so then your baby needs more formula, ect. its a vicious cycle that is tricking your body into thinking its doing a great job, but if only it knew it needed to make more, it would.
for nursing twins, although i dont have any, i would suggest the breast for each. your body needs to have consistency, so if one baby is always eating an ounce more, you want the same breast so its always prepared. i also say always feed them the same time, otherwise you will wear out.
just reading your post, i know you can do this:) just let your body figure it out. aside from all that, i think you need to eat more. and drink more water too. i know the amount of extra food i eat, and fluids for one baby, i would have to assume you need much more for twins. its probaly more calories than you want, but think it will def help you. good luck, and know you are doing a great job, i just hope you realize that yourself!!!