[KenWittlief]

the only problem I see is hooking generators in parallel. They will not be perfectly balanced, so one of them will motor the other to some extend.

Hooking them in series is not ideal either, because what you get from a generator is not really pure DC. Its more like a rectified AC waveform. This is due to the commutators/brushes, and the many windings. It would be very difficult to attach two motors to one drive shaft such that the windings and commutators are perfectly synchronized. If you have a two channel scope, and you hook each generator to separate channels you will be able to see the two rectified AC waveforms, and most likely you will see they are out of phase (out of synch).

But that can be fix to some extend by putting caps across each generator - even better if you can put power diodes infront of the caps so they dont back feed the windings.

by now you are saying YIKES! this is more complicated than I thought. Best answer is to use one generator.

The other part of your question: how much power can they put out? Quick answer is to try it and see. Voltage is only half the power equation. If you can load the generator up with power resistors or a bunch of 12 light bulbs (like headlights) you will get a quick indication of when the generator starts to droop its voltage, and how hot its going to get after pumping out real power for a while.

DC motors tend to be in the 90% effeciency range, so a motor that is rated at 1/4 HP (180W) will require 200W to run, and you might get 162W out when you use it as a generator.