I think this needs to be done in mame controls. The problem with doing this in retroarch is that is built for 2 sets of controls. In this case I have one set like in the arcade, I just need a 2 player start button. Only mame is going to know about this.
Is it possible for me to 2 set 2p start hot key?
input_player1_start_btn
input_player1_start = "ralt"
# input_player1_start_btn =
input_player2_joypad_index = "0"
input_player2_start_btn = "0"