That's an interesting case; and your hypothesis is certainly plausible. I can see no way to affirm or deny 1 case: a few
other hypotheses occur to me.
A) accepting monozygosity
1) Perhaps a virol [sic] or toxic injury to one fetus preceded (and caused) the circulatory difference: a retarded fetus may
evoke less than vascular supply.
2) Postnatal stress may be the immediate modifier of the hypothalamic homeostat, whatever the etiology of that stress (dysnutrition
. . . )
B) questioning monozygosity
(The data as given are thin. What were the parents' blood groups?)
1) H's failure to thrive reminds one of RUNT runt d[ . . . ]. That is, he may have been a Chimaera with some "graft
vs. host" immune reaction [mark to note, or "freemartin" effect]. Chimaeras can be hard to diagnose My colleagues
at U. Wis. worked on runting in dizygotic twins in animals. I might be able to dig out some of that literature if it's
not already familiar to you.