Hi J. -
Even though your girls are twins, they are still quite different, seperate little individuals (as I am sure you know!) that are at a developmental age where they are starting to be aware of their needs as seperate from everyone elses. Just because they CAN sleep through the night doesnt mean they WILL. More and more they will start to exert their want's and needs regarldess of what you or anyone else does - they are girls, afterall.
All you can do is be consistant. Make them aware that there is a schedule, and that you are the schedule keeper - feed them at the same time every night before bed, and if you know they dont need the nutrition of a bottle at night, when they wake up, give them a bottle of water as opposed to milk, or just rock them until they go back to sleep. There are several times in their infant/toddler lives when their sleeping schedules can go through drastic, but normal, transitional changes: this is one of those times. Once their bodies get used to knowing that they dont need milk at night, the sleeping will take care of itself as long as you keep the routine.
Good luck!
K.