I honestly think that there are more than two types of hypo bulls out there. Genetically what is going on, it seems like no one may know. For instance, I have 4 hypo bulls:
Stillwater hypo ( sorry for the bad pic )

Hypo bull ( line unknown )

Sherman line hypo

Outcrossed Iowa hypo bull

All four of these types of hypos have their own look and set of characteristics to them. Since the Stillwater hypo gene was not compatible, according to Shannon's breeding results, my first thought would be that if any other hypo lines were bred to different hypo lines, then the results would be the same and show as non compatible....but...I could very well be wrong.
If it were me as far as breeding hypos, I would personally stick with breeding the hypos to the same type of hypos. That way, there would be more of all the different ones in existence.
There is so much to learn though about hypo genes, and perhaps breedings in the future can show us some answers.
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Genesis 1:1