A common question I get is "How can 2 players play on the arcade?"
You have two options:
1) plug a USB controller into the back and map it for a second player. OR
2) do what Jimmy or Martin did and mod a fight stick(or build one) to plug into the RPI's extra GPIOs for player two controls. Thank you to Jimmy and Martin for documenting and sharing their ideas!
Martin did a similar 2 player mod using parts that are more DIY friendly (a DB-15 breakout board, sans the raw DB-15 connector) and with a simple enclosure- a wood tea box found on Amazon. The write-up is well documented with links to all parts. I highly reccomend giving it a read if you want to add a 2nd player control box to your mini arcade.
Jimmy used a fightstick and broke out its controls to a D-sub connector. I will let him explain...
"First thing I did was cut larger one of the openings on the back panel to fit the D-sub connector. I then bought some header pins and soldered them on the back panel D-sub so I could quick disconnect the panel without needing to plug back in the individual connectors on the pi. The joystick I use in the pic I bought online but it had a USB connector so I gutted it and replaced the buttons and redid all the wiring so they connected directly to the pi. With 5 pins for the joystick and 8 more for the 7 buttons (6 play buttons plus one more for 2 player start), I was able to use a 15 pin D-sub connector."