Different kinds of hardening abilities
There is no such thing called &quot;white&quot; hardening, Warhammer hardening is crystal type, like female or attack titan, and Jaw hardening is neither natural nor white(crystal), but it looks like the natural type. so because armored titan is the only titan that can do natural hardening, there won't be any hardening combination.
EDIT :
I was wrong about Warhammer's body, it IS hardening but I don't think it's different from other types of hardening