I'd say shared variation sounds weird mostly because if there were two characters together like Noob-Smoke, or maybe even two characters fused into one, it'd be cooler to see those characters use their powers combined instead of isolated.

I mean I guess it makes sense since Cassie has two variations inspired by one parent specifically but having too many characters utilizing past characters will get old really quick. And in her defense she at least has moves from both of them that are independent of her variation.

Ferra/Torr already have the tag thing going on and Cassie has the fusion thing so honestly it sounds like MKX already has the shared variation theory.