Okay, for future people, here's what I've figured out:
To double jump, start a scan at the height of your jump. Then, whenever you want, hit jump BEFORE ending your scan. You want to end your scan something like 1 or two frames after you hit jump. You don't actually need to push jump after you end the scan, just before it. As far as I can tell, you are limited to 1 double jump per jump (i.e. no triple jumps) but idk for sure yet.