So the reason you can't do that right now is just because of how Godot's input system works. All of the available inputs for hitting a note are assigned to a Godot input, but that means that if one of those assigned buttons is held down, pressing another one won't activate the just_pressed() function, so it doesn't count. I'll see if I can figure out a workaround. I'd like to eventually implement a key rebinding menu, so then you could just set your own 2 inputs for each key as separate Godot inputs.
Some binding that might seem odd, because they're just bindings people asked me to add (A&D, F&J, S&K), for until I do get that rebinding menu added.