Xenophobe is the only one that I can think of at the moment that actually used two.
M.A.C.H. 3 had two, but only used one, IIRC.
Xenophobe can be played very easily with buttons on the CP though because you don't need the other hand for a spinner.
The games that you need a spinner in the other hand (Tron, DOT, Mad Planets, Zwackery) don't need more than one button.
My suggestion is that you should put one button on each, but put them so that one stick will be a left hand fire, and one a right hand fire.
That will let you play tank games with them, notably Sarge and Vindicators.
I think Xenophobe actually plays
better with CP buttons.
Pretty sure Mad Planets had two thumb buttons, but I can't remember if it actually used both of them. I can't remember if MACH 3 used both buttons, either.
Don't forget Battlezone and Assault on the tank game list! 4-way restriction would be nice for these, but you can't have everything.
DOT and Zwackery likely
do need extra buttons; unless pcolson has an up/down spinner he'll need two buttons to perform the actions the up/down portion of the spinner ought to be taking care of. Those, in addition to the one thumb button already needed, would make for
three total thumb buttons. I don't know if you could
fit 3 volcano-type buttons there, but one could probably find something else that would work. Or just put two buttons on the CP and live with the fact that sometimes you just have to take one hand off a control.