[A_Reed]

You can add the power of the motors in each gearbox. Power is absolute and additive and if you know the relationship between power, torque and speed (P=T*n (with proper units/conversions)) you can define all of the characteristics of your motors in your gearbox. Your power will remain the same as you match output speeds of your motors but your output torque will go up by the same factor that your RPMs fall.

If you look through the CD white papers you can find a topic on 'traction limited' drive trains and what maximum gear ratio can be used so that you don't trip breakers and your wheels don't slip. It suggests finding the power of your motors at a maximum 40 amps of current draw so that you stay within the limits of your circuit breakers.

As far as max speed and max acceleration go, that is something I have been working to understanding myself and I have a spread sheet made up for my own understanding of the topic. Maybe that topic can be answered by someone wiser than I.

JVN's drive-train calculator is also very nice and I recommend it for quick design checks, but it can be a little bit of trouble when you start mixing motor inputs. I wanted to see how the numbers worked so I did my own math and checked it with JVN with mixed results based on my own design assumptions.