Purely because of the number of comments, I'm not going to reply individually, but thanks all for the comments, and here are my thoughts:
I agree that if you can toggle straight to willpower, you cannot toggle back to stamina - you have to stick with the willpower bar.
However, another idea I've had is that actually they display different things, as a few people have hinted at/suggested. The stamina bar is indeed for all stamina actions, and when it is used up, you then go into exhaustion. The willpower bar is separate and does display your morale, and is basically a "modifier" for the effectiveness of all your stamina actions. So the more fearsome your foes, the longer the battle goes on for, etc etc, the lower your willpower bar might become, but it can also go upwards, and is separate to the stamina/exhaustion bar.
So, I see three options, I think:
A) Stamina -> Wpr -> Exhaustion. First standard, second stronger, third weaker. Exhaustion cannot be 'avoided' once you dip into exhaustion.
B) Stamina -> Exhaustion/Wpr -> Exhaustion; you can choose to enter the Willpower stage whenever you want. If you don't, you default to A, if you do, you skip the rest of your stamina until you next recharge your exhaustion.
C) Stamina -> Exhaustion, and the Willpower bar is totally different.
Additionally, if A/B, currently you can stop any point in your stamina bar and recharge; that's fine. But I could make it that once you're into the willpower bar, you *must* finish the bar, i.e. it goes down no matter what, and plunges you into exhaustion no matter what. I think I'm leaning towards B) at the moment, but as I say, it's going to be a while until these really get used! But yeah, I think B sounds good, combined with "once in willpower stamina you cannot escape exhaustion". Equally, to stop people just automatically flicking into willpower for really brief battles, maybe make something like each time you enter willpower stamina, your exhaustion timer is increased by 1 turn? That's tiny (player's average speed is one move per ten turns) but something LIKE that might add up and be a good idea.
So, if we stick with B, how about: Stamina leads normally to willpower, which is then guaranteed to lead to exhaustion. You can choose to enter Willpower for a big boost, but once in willpower, you again cannot escape later exhaustion. Every time you enter your willpower bar, your exhaustion timer is very slightly increased. This gives you a lot of control over the timing/pacing of the battle, allows both endurance-focused and willpower-focused characters to quickly make use of their abilities, and disinclines you from abusing the willpower bar (maybe higher willpower actually reduces the exhaustion increase each time you use it?).