Those are the only two options I consider because any partially balanced choice is (quite highly) inferior.
I agree that the present inferiority of a mixed strategy is a problem. But I don't agree with your proposed solution.
Decouple the sliders and you finally have a reason to do something other than all-X without deliberately hamstringing yourself. Sounds like that opens up a lot *more* strategic decisions to me!
Not really. If you decouple the sliders, then the obvious winning strategy is to go all-x on both sliders, which is a no-brainer.
Also, you'd have to specify how you want to implement this decoupling in game-terms. The sliders allocate your budget. You can't allocate your whole budget twice, so you'd need a more fundamental change to the way the sliders work. There are several possibilities, but each of them introduces its own set of problems, while not actually solving the underlying problem.
Imho, the underlying problem is the curent inferiority of mixed strategies. *This* should be analyzed and addressed. A decoupling of sliders would just introduce new problems without actually solving the ones you mention.
But anyway, we've digressed far from the purpose of this thread, which IIRC was to collect ideas rather than discuss them. We should take this issue to a dedicated thread if you like to.