1) is it worth buying 2 spinners? (probably slikstik tornados)
Definitely.
The tornados are a bit on the expensive side though.
They also use their own interfaces.
I'm not sure if there are any issues to using more than one because of this.
You can replace the optics with Oscar's, but that kind of defeats the purpose of spending more in the first place.
2) what mame games would have p1 and p2 both using spinners at the same time?
Blasteroids, and alot of driving games.
3) would it be simple enough to piggyback the signals from the spinners onto the x-axis and y-axis of the KE72-T Trackball encoder?
It should be no problem using Oscar's switching circuitry, but I'm still not sure about using the Tornados.
I seem to remember some people having problems interfacing the stock optics on the Tornados for some reason.