That's a lot of code to modify, you must change various things, in player.wdl, for example, IF (my.movement_mode == 2) { my.jumping_mode = 0; ..., i guess this means if you're already jumping and press space again, it will ignore space.
then you must change also the animations. I guess it's a lot of work, and maybe you should post this in "user requests". Good luck.