Board Thread:Theories and Speculation/@comment-25643824-20161215211806/@comment-734582-20161216114242

Tasuxeda wrote: Yes, Madara thought that just mixing Senju cells with Uchiha cells was enough to get the rinnegan. He didn't even know that. Madara interpreted the Stone Tablet as "when Uchiha and Senju merge, they awaken a power beyond either".

Considering they were long-time enemies it's pretty much inevitable that they have tried to steal each other's powers before. But it only works with Transmigrants and even then, it seems to require a fully-realized transmigrant.

Now a Senju-Uchiha combined clan may be pretty powerful, but it also may not at all be. Keep in mind that many KKG were lost simply because they were bred out of the population (wasn't it part of the backstory that many other clans had Byakugan?) or forcibly removed (IE the blood mist's war on KKG).

Hashirama himself is also the poster boy for "having a KKG doesn't guarantee your kids or grandkids to have it"