i think a major update that should occur with this is the ability to hit multiple keys on the keyboard and get an input for each key. (i.e. hitting two left keys at near-same time, and getting two left inputs) because presently hitting a key leaves a TON of dead space where another key of the same side cannot be input. this would help a lot with things like double taps not being as finger-breaky, and would just make the game feel better (not to mention there are some wierd controls. f is on left, but d is on right, where j and k are both on left??? it doesnt really make sense)
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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.