Yeah - the jump command is pretty specialized. But this is totally achievable with just blueprints! The Swing Ninja sample from the Marketplace should give you a pretty good idea how to implement double jump. Essentially instead of calling jump, you can just use the AddForce blueprint node, to impart the physical force you want and as often as you like.
Alternatively, you can set the Velocity of the CharacterMovement component directly (which is what swing ninja actually does)
Cheers,
Nick (Epic Games)