Very good find, will save a good chunk of time here for sure. Here's a way to do it without double jump:

I just got this is well, but it was when they attacked by slamming their heads together. I was under the right head then, but you got it from the left side so I guess it doesn't matter.

I believe I got it when the attack put me at critical health (when you hear that distinct sound effect) and it looks like you got it then as well. Just speculating, but maybe your hitbox gets certain properties when that happens, so collisions get wacky?