Turns out it was easier than I expected it to be. Here's my Solution in case anyone is interested:
1) Add Boolean instance variable "DoubleJump (False) to Player
2) +Player On Collision with Goomba
- Player.Y < Goomba.Y -> Goomba Destroy
Set Boolean "DoubleJump" to True
Player: Set Platform vector Y to -400
Touch: Any Touch Start
OR
Keyboard: Any Key Pressed
: Player is DoubleJump -> Set Player Vector Y to -750
Set Boolean "DoubleJump" to False
3) Player On Landed -> Set Boolean "DoubleJump" to False