G.K.
I myself didn't have twins, but I'm a peer counselor with Nursing Mothers Counsel in the Bay Area, and we have several counselors who have worked with moms of twins. We are a non-profit organization of moms who have breastfed and gone through additional training to become peer counselors. We provide FREE breastfeeding classes (although they are here in the Bay Area) and one-on-one counseling, mainly by phone, to help support YOUR goals for breastfeeding. You can get more information at http://www.nursingmothers.org.
That resource being offered, TAKE ADVANTAGE of the lactation consultants where you deliver!! They should be a wealth of information especially in terms of latch and what to expect. Try to take a breastfeeding class in advance and ask specific questions about twins. I know moms of twins who have supplemented, and moms who have exclusively breastfed. Supplementation could be recommended by your pediatrician, or it could be wanted by you for a little break from the babies.
Whatever you decide, understand that for 99% of women, whatever your babies are demanding from you, you will provide! Have lots of support for when you come home; people who can clean, cook, do laundry, hold babies when you're not nursing, feed if you decide to go back and forth between breast and bottle/cup/spoon/etc, run errands, help with your 2.5yo. Surround yourself with people who will HELP and people you feel comfortable with :)