I didn't work on cracked fusion. But I imagine it wouldn't be much better. Like I said, two types of Gem aren't supposed to fuse, because you lose who you are supposed to be. Pair that with a Gem that's already damaged, and you'd get something entirely out of control.

Intact Gem fusion of the same type is allowed only in specific scenarios. There isn't much to experiment with there. You just get a more powerful version of the base Gem.