Double jump with variable jump

How do I...?
#1

I want the user to have multiple jump heights (the longer you hold the button, the higher your jump) while also being able to double jump. (like in I Wanna Be The Guy)

I haven’t found anything about this. So, is it impossible? is there no way to do it?

#2

Variable jump height is in the platformer settings, called “jump sustain time”.
Double jump is done with an action called “allow jumping again”. There are probably several tutorials about this.