An intriguing premise, but in practice I think the novelty doesn't pay off. It took me a minute to figure out that Defend blocks damage from the healing mechanic too, which I didn't expect, but that 'discovery' was about all the learning curve I faced. Despite the description, it didn't feel like a tool that turned the tides of battle, so much as a standard heal that took two turns, one of which was a Defend turn. (This could theoretically play out differently with the ally, but it has so little health that it almost always died before I could test this out - being dead in one hit against most enemies without a Defend action, and useless if he just Defended every turn.)
I wonder how this would play out if the space between healing and being re-damaged was longer, there was no way to mitigate that damage, and some other mechanical change - like increased armor or damage - happened when healing as well. IE, once you 'heal', you've got a ticking clock before death that can't be stopped, but also get a big buff to really make those last handful of turns more interesting. I'd love to see this idea explored more, even though it didn't land for me here.