I'm planning to build an Arcade-cabinet from scratch (using WinXP and MAME for Windows), so I want to know the following:
1. Is it possible to use 2 Trackballs at the same time, to play i.e. Marble Madness using the 2 player mode?
Not with winXP. You either need to go pure dos & dos mame, or win98/winME with two USB trackballs & mame:Analog+ (see my sig, below, for link)
2. Can I do this with two USB-Trackballs too?
Only with win98 or winME & mame:Analog+
3. Which interfaces support USB? Only Happ-control?
Like SirPoonga says, it's either happs or hack a USB mouse.
4. Does anybody has experience with the USB-Happ-control interface?
Just got one, works fine on my computer. I have a happs 3" connected with the happs USB interface and a USB kensington trackball (2 1/4") working great on two player Marble Madness with mame:Analog+ and winME.