First, the CIMs are 5500rpm, not 4200.
http://www2.usfirst.org/2003comp/Specs/Chiaphua.pdf.
Second, the breakers will not let you get 120amps for a single motor. That is limited to 40. The 120 is the total number of amps the entire robot can draw.
Third, you need a MUCH better reduction than 3:1. With 9" wheels, you probably want a wheel speed of approx 250rpm. This can be accomplished very easily be using the gearboxes FIRST provided with the drill motors in low.
Fourth, until you get a few years under your belt and have been to a few competition to see what works on what doesn't, I wouldn't even recommend attempting a two-motor transmission. Along those same lines, when you do go to a regional, assign at least one person to go talk to other teams and see how they did their drivetrains. It should give you some good ideas for next year.