ah the infamous 2 wheel drive topic... my high school team, team 263, has used 2 wheel drives ever since 2001. In 2001 it was a 2 wheel front drive, 2002 was 2 wheel center drive, and 2003 was 2 wheel back drive. The difference we did was, no casters... we used teflon skids. In each place you would put a caster, we put a curved peice of teflon since that was the best material we found with low friction that didn't get scratched up easily. The skid system won us a Delphi award in 2001 and a Motorola award in 2003, and our 2001 robot also won the long island regional in 2001. 2 wheel drives provide you with a much greater ease in turning, but thats pretty much the only advantage you can get from them. With the concept of having a caster or skid on one end or both ends, you will have weight resting on these skids or casters, thus casuing less weight to be on your wheels which ultimately leads to a lower max pushing power. Some recommendations I give if you are set on making a 2 wheel drive would be:
1. keep as much weight as possible over the wheels, ie. battery, compressor, add iron under the axel to bring the robot up to max 130 pounds when done building, etc.
2. find a low friction non-caster material for the skids, avoide casters at all costs. especially if we get a field element like the ramp mesh from 2003.
3. add a wheel front and back of each center wheel to make it a 6 wheel robot
In all unless you get a game like 2002 where you can grab 2 goals and balance your self out over the 2 wheels, I would not recommend a 2 wheel drive robot. However, it can lead to some very interesting design ideas and innovative awards, with the added bonus of easy driving.