1. The cabinet controls are plugged in the Raspberry Pi via USB using a “DragonRiseInc.GenericUSBJoystic” controller like this one: http://pt.aliexpress.com/item/Zero-Delay-Arcade-USB-Encoder-PC-to-Joystick-Control-panel-For-MAME-2pin-SANWA-Type-Push/1045954113.html
2. Its single-player tabletop cabinet and I found easier to add a second control for casual playing by plugging a USB SNES controller than building a new cabinet or control panel.
Anyway, the cabinet controls are normally identified as Joystick 1 and, as I said it already works as so with all console emulators.