First of all, you are my hero for breast feeding twins. There is a possibility of my having twins, and I would be determined to do the same, but I know that it's not easy and a lot of people give up- But I keep thinking about the 1600 extra calories (I really like to eat:). Also, I know how hard it is to make the decision to wean off, even just a little. Don't feel bad about it! You breastfed twins for 5 1/2 months!
I would try switching your formula and your bottle until they take it. I had a hard time when I weaned Izzy (she had issues with staying full- at 6 months she still fed every hour, so I started giving her two bottles a day to keep her more satiated, and then she decided she didn't like my boob anymore!). I had Avent bottles and the samples of Similac when I started- turns out she hated both, plus the Similac constipated her like crazy and made life miserable. I tried Good Start and switched to the Playtex drop ins with a latex nipple, which I suspected she would like more for two reasons- the playtex bottle has a wide base, so she could (and did) latch on like she did on the boob, and because she always prefered latex to silicone pacifiers (I would imagine the latex feels more like skin). Kids are fickle. I would suggest the latex Playtex nurser to you (the latex is the brown nipple, not the clear one) since your babies are used to the breast, too. And Izzy did the best with Good Start, then generic formula (Generic is really the best- it's cheaper and just as good. Often it's even made in the same factory as the name brands! I used the Target brand). Good luck!