We already implemented fire button two for jump. Make sure your emulator is set up in a way that fire button 2 is assigned to a phisical button on the joypad and you should be good to go.
I probably wrote it unclearly. Fire2 works for me (real C64 and emulator), but.. Accidentally holding down direction UP blocks the use of Fire2 for Jump. And vice versa. This affects when playing with a gamepad. And not important when playing one-button Joystick.
I use gamepad on real C64, USB device to DB9 adapter. Fire 1, 2, 3 works.