Making a 2D character jump in Unity is relatively easy, it's simply a case of adding upwards force to their RigidBody component. A double-jump however isn't quite as simple as adding another force on the second button press. Doing this will cause different behaviour depending on how closely together the player presses the jump button, pressing it in immediate succession will cause the force to cumulate into a big jump, pressing it at the peak of the previous jump will barely have an effect on the player's jump height. In this regard the numbers can't be tweaked to recreate the way jumping works in Super Smash Brothers.

The solution I settled on was to have a Vector2 of the player's jump target, which, upon pressing the jump button would be set to the player's position plus this jump's height. The player would then attempt to lerp there, ending the jump if they collided with something they couldn't jump through.