There's certainly no point to having TWO stances. If you just make one of those the default, and the other one modifies your defense/attack in relation, that would already simplify things by 50%.

But then you have to wonder if having this be a toggle on/off is even worth it.
Making the effect of the stances their own abilities, even on cooldown, might be the better thing to do.
At the very least, the stance could simply be a mechanic of one of the tanks, it's unique mechanics, and a way to differentiate them a bit more.