Combo Action Button

[Reynhart]

How would that be complicated? This exact system already exists for Fel Cleave and Inner Beast.

That situation doesn't exist for now, with two skills sharing a slot while not sharing a CD, so I don't know what would need to be changed to create something like that.
This would also make tracking the CD for each of them impossible without changing your stance. Like I said, you don't have this issue with GCD precisely because there's no cooldown for them.