If your clinic did not provide all prenatal care, and you have to split the billing, then you bill the 59514 or 59515 for the twin C/S delivery. Usually. Most insurance carriers won't pay for delivery of the second babe when both are delivered via C/S. If both are delivered vaginally, or if one is vaginal and one is C/S, then you can bill 2 delivery codes, but most carriers feel the amount of work involved in delivering the 2nd babe via same C/S incision as the first is not significant enough to warrant separate reimbursement.
Check your carrier guidelines, though. I have one carrier here in Colorado that will pay for that 2nd C/S delivery, but most carriers won't. If the additional work was significant, I have to go with a modifier 22 on the 59514/59515 code, then appeal it when we don't get any extra reimbursement, and THEN argue with the claims processor who denied my appeal as a duplicate claim. But maybe that's just me. ;-)
Becky, CPC