Undies, my prayers go out to you.
Basically, one twin is getting more blood out of the placenta than the other. One may be born smaller, anemic, and pale. The other one bigger and redder (due to more blood).
I know this because my wife's doctors thought twins had it, but it was never diagnosed. They were monitored and kept close enough size wise until my wife had the C-Section. One of the twins was "plugged in" near the center of the placenta, and the other one was "plugged in" near the edge.
From what I remember, and what my wife helped me remember, there's a hospital in the SW (AZ or NM, we believe) that specializes in this.