I find the idea itself interesting, but it will mostly depend on the context and how it interacts with it.
I mean: if you're in a classical JRPG and it's basically a Final Fantasy but with the addition of the healer (which is typically a weak character) losing HP when healing, it might not be the best design choice.
Now, if you're in some kind of coop bullet hell where you have to not just do the typical dodging and shooting but also you have to be administrating health points with your partner, that's an entirely different thing.
Then, if in that coop bullet hell both players have a special canon shot that makes you lose health points when used, it adds an even deeper strategy level.
If you add also enemy types that are weaker to the special shots of one of the two players, you have even more strategic choice built around the self-destructive healing mechanic.
As I said, it opens a lot of possibilities to build a strategically interesting rule/mechacnics set, but without the context where that mechanic is gonna be applied it's hard to give a precise answer