You can plug in up to 2 X-Arcade™ dual controllers or 4 X-Arcade™ Solo 1-Player controllers on your PC or Mac for up to 4-player play. You can connect all of them into USB ports, or plug one X-Arcade™ into the PS/2 port and any others into USB ports. You cannot plug one X-Arcade™ into another.
Dual/Tankstick: You will want to simply leave one controller in Mode 1, and program each extra controller to use different keys than those used in Mode 1. For 2 trackballs the software must support this, see the MAME™ help section for using 2 trackballs in MAME™.
Solo: For 2 Solos, it is easiest to simply make the 2 Solos act as a 2-player X-Arcade™. To do this, you'll simply want to program one of the Solos to be the player 2 controller, leaving the other in Mode1. Program the player 2 Solo to use the same buttons as the 2nd player side of our 2-player controller as shown in the image below.
You can use 2 console adapters to play 4-player on consoles in general. For consoles like the PlayStation 3 and Xbox 360 that may only have 2 USB ports you can simply use a USB hub.