I've read about problems having both hooked up to the computer as a USB mouse. Can I have both hooked up via USB at the same time and just use the one I want for a given game?
You can hook both up by USB at the same time and mame can see both mice. However, mame now treats them like joysticks: each mouse device is read seperately. So much like if you have a steering wheel and a gamepad hooked up at the same time, you will need to do some remapping so driving games use the wheel and others use the gamepad.
This is easy. Discover what each device is called; you can temp map in any game with analog inputs and note the numbers. Once you know the mouse numbers, use "Input (general)" to remap all of player 1's trackball ports to read from your TB device (whatever mouse number it is) and dial ports to read from your spinner.
You might want to save this as a
ctrlr file, but this not needed. But even if you don't ctrlr it, the one remapping will stick for all games, and you'll be able to play like you want.
My other option is to get the Opti-PAC version of the spinner and get a different trackball and run both through an Opti-PAC. Or can you hack the X-arcade trackball to work with the Opti-PAC? What trackball is preferred (besides the HAPP, I'd have to convince myself its worth the money first)?
IMO, happ has the best, but followed closely by betson/imperial; I tried an Xarcade TB, though. However, I wouldn't worry about trying to combine the devices into one mouse since mame can handle the mice devices seperately.