Okay, HK, I got it figured out.
There's no need for additional codes on your end.
My CP is configured and set up in MAME as such for 2-player 8-way Joystick games:
            ***PLAYER 1***                                                          ***PLAYER 2***
Joystick 1     Buttons     Joystick 3                                   Joystick 2     Buttons     Joystick 4 
I had to map a label for Player_1_Joystick to both stick graphics for Joysticks 1 and 3.
I had to map a label for Player_2_Joystick to both stick graphics for Joysticks 2 and 4.
I had to map the labels for Joystick 1 (up, down, left right) to JOYCODE_1_SWITCHes (xaxis, yaxis)
I had to map the labels for Joystick 3 to JOYCODE_3_SWITCHes
I had to map the labels for Joystick 2 to JOYCODE_2_SWITCHes
I had to map the labels for Joystick 4 to JOYCODE_4_SWITCHes
So basically the joysticks only utilize player 1 and 2 joystick controls, but their labels utilize joycodes for 1-4.
Again, thanks for a really robust app.