I found this online. Perhaps it will be helpful. It's a discussion between Marvin Shilmer and Ron Rhoades about blood. I will paste the link and cut/paste the pertinent section. It's the only place I've seen this issue discussed where a witness viewpoint is given. The society has said nothing on this subject.
Link: http://www.geocities.com/rogueactivex/RRhoadesvrsMShilmer-P2.htm
CUT/PASTE SECTION:
DMT describes unborn twin babies that have their own placenta and their own circulatory system but share some vessels between each other. Throughout the pregnancy whole blood is transfused via these shared vessels back and forth between the two babies. So, though each child has its own circulatory system (just as does a mother and her child) in the case of DMT they actually transfuse whole blood back and forth throughout the pregnancy. Of course, when whole blood is transfused between a mother and her unborn (as does sometime occur) this could cause death because of rejection of foreign tissue. But the blood of these twins is identical because they are identical twins; therefore the blood transfused back and forth does not usually cause problems because it is perfectly matched just as creation would have it...Therefore with DMT babies we have in creation a perfectly healthy and naturally occurring instance of whole blood transfusion in the 85% of occurrences of DMT cases where there is no TTTS. If we follow your premise that blood parts transferring naturally according to Jehovah's creation are maybe not "blood" that we must abstain from since this process is naturally occurring, then we must conclude that transfusion of whole blood is maybe not "blood" that we must abstain from because we see it occurring naturally in cases of DMT babies.<
There are several valid reasons why this cannot be considered a parallel to anti-bodies/fractions being passed from a mother to her baby. First, we are actually looking at essentially *one* circulatory system not two as you assert. The blood is not removed from one and introduced into a foreign blood stream, but "naturally" belongs to both twins. There is no interruption of the corporeal circulation in utero. This being the case, we do not have a parallel to fractions being passed between two disparate systems. This, then, can not be used to override or modify God's command to abstain from blood, which when *removed* from its body can only be used for sacrifice or poured on the ground. Therefore your example is not relevant.
As you note, even the DNA is identical in identical twins' blood as well as all inherited substances. The fact that problems occur whenever major components of blood are passed from mother to fetus only reinforces the decision to include major components in the Scriptural ban while fractions may be reasonably excluded.
Since the above is true this second reason is largely unnecessary, but still reinforces the lack of parallel in our examples. The question is whether DMT are a "normal part of God's creation, or rather a defect. I think it can be argued that a "normal" twin pregnancy would be when the egg separates early (by 3 rd day) and both fetuses have separate amnions and chorions. In this view, a shared chorion would be considered a departure from creative design and the result of imperfection similar to the mingling of mother and baby's blood. Lending sound evidence for this is the fact that progressively later (abnormal) separation of the egg leads to greater and greater physical abnormalities and risks culminating in conjoined twins (after the twelfth day). If this conclusion is correct, your example is definitely not parallel to the passing of fractions--an undeniable part of creative design. There are no increased risks with fraction transference like there is with shared vessels in DMT.
