While I was struggling to make my own custom Platform Movement, I decided to try to make a simple double jump using the default Platform Behavior.
Since I'm still a noob at this, I don't know if this is the best way to do it, but to me it seems really simple and gets the job done.
Develop games in your browser. Powerful, performant & highly capable.
Nice job. Simple yet effective, and also easily adaptible to triple jump and beyond... even other applications like for example, simulating a character lifting a heavy door where you have to bash the button over and over until it's at the top or else it drops to the grond and you have to start again... etc.
jump sustain doesn't work for the second jump
can compensate by temporarily boosting jump strength, but it's still not ideal (don't have that level of height control over both jumps).