User blog comment:Khalael/Canon Policy Discussion 1: Space Marines/@comment-2253300-20131106083600/@comment-1620907-20131106084430

I'd also like to add the point that mixed geneseed chapters would likely only be created when stocks are thin and you need astartes quickly. Otherwise the risks involved would appear to be too high. Especially when considering the instability of some gene-seed that Supah has brought up. Using two stable gene-seeds could hypothetically work, but adding anything unstable in there would just be asking for major trouble. I also agree that if dual geneseed is desired, then it shouldn't just be as an excuse to make Ultra Fists or Blood Scars. There should be some imaginative thought put into making them an individualistic chapter.