Ok, now that I think about it, it could get complicated. I would have it be any non-direct offense spells or permanents like Reverse Time and Bone Shield would separate and destroy (or whatever) the rider. Offensive spells like fire/ice bolts and poison effects would kill both creatures when the fusion card died.
This could be a standard rule for fusion creatures if they were introduced to the game. Just an idea.
Oh, that's what CC means.
