Can these be stacked (bolt on another throttle and gear shifter)? Or do I need to buy a third throttle control and gear shifter?

Yes, they can be stacked, you may need to buy or make longer cross rods though to hold the stack together.

Yes sir, just call them up, you can buy whatever parts you need factory direct.

I have the parts you need used, white in color. 1 extra throttle handle (long) and 1 extra shift handle (short). With rubber boots. Removed from a triple throttle to make a twin setup. I'll have to dig but might even have the longer screws that you'll need. PM me if interested...