A sex act in which, the male(s), while fucking twin females, cum into the mouth of one twin and she then spits that load into the mouth of the other twin.
A play on the actual condition in which during a twin pregnancy, arterial blood from donor twin passes through the placenta into vein of recipient twin. Essentially the donor twin gets less blood and the recipient twin gets more.
Man I had a great time with those twins yesterday. They even performed a twin-twin transfusion.