Hello,
There is no easy way to do it at the moment but to put an invisible cube under you while you jumping in mid-air and then make it disappear after you jump.
Another issue is to know when you hit the ground to let the player do a double jump again.
We found a workaround but the problem is you have to check on each object in the game if the player hit it in order to reactivate the double jump.
We will try to think of a better solution or add some support for it.
Here is the example:
www.gamemaker3d.com/player?pid=01632389
Regards,
Cyberix3D team