What is the incidence of mixed delivery (one twin delivered vaginally, one twin delivered via c-section)?

About 4% but rising: The most common reasons for C-section of a second twin are heart rate problems and the baby coming down in a position other than head first. Many doctors used to be willing to deliver a second twin either butt first or by the feet, but few still have much experience in doing this. There is also a risk of injury to the nerves of the arm or learning disability in any baby that is born breech.

IT should be very lo: Low as the only twins that should be delivered vaginally are if they are both head first, if either one is not head first then u need a c-section, and many docs will only do c-sections even if both twins are head 1st, if the first one comes out vaginally then it is an emergency csection usually because there is a problem which u want to avoid delivering the 2nd one.