WOW! I LOVE a twin photo shoot! Even though it can be such hard work with 2 babies. And no, I charge the same price for twins as I do for a single baby. With a twin pregnancy, due dates are even less reliable that with single baby and most twins are likely to arrive early. I would do a twin shoot as soon as possible after both babies are home and well enough. Twins are generally smaller and because of this and an earlier arrival, the time frame in which to photograph them at the optimal newborn stage is usually longer than the normal 2 weeks. Just keep me updated on your and their progress along the way.
Further, just to add, I am a qualified Midwife and a qualified Neonatal ICU Nurse and so I have worked with pre-term newborns a great deal of my life. I am very comfortable handling tiny babies safely.