I miss "Tom/Zach." Where is he? He knows tons of stuff about that, and maybe he could educate me.
Gee Farkel, I'm touched.
Actually, Hawk and Maximus have done a fine job and there's not much more to say. As monochorionic (both monoamniotic and diamniotic) twins, triplets etc. develop and grow, so does the placenta. Pretty soon there are scores of intersections as the network of vessels from the umbilicals of each respective twin become more and more intertwined. Their blood is freely exchanged. This is quite common.
Less common is twin to twin transfusion syndrome (TTT) where one twin, the "recipient" gets more than his or her fair share of the blood, while the other twin, the "donor" is robbed. The upshot is that one twin is born with an overload circulatory system while the other is born severly anemic. Both conditions are life threatening and require immediate attention. However it is at this point were current WTB&TS policy rears its ugly head, because an important and sometimes only means of saving the anemic twin is forbidden to Witnesses.
Tom