> I don't think that two opposite spinning gyros would cancel out,

If you try to tip a spinning wheel, it will respond 90deg later. Try it with the front wheel of your bike. Take it off the bike, give it a spin, and try to tip it so the top goes one way or the other; the front will move that way instead. If you had two wheels spinning in opposite directions, one would take that response forwards, the other would take it backwards, and they would cancel out. Guys trying to power cars with flywheels are using the same approach. Imagine the dynamics of a car with a single flywheel spinning at 100k rpm or so.

Anyway, it is possible to operate bikes, airplanes, and other craft which have negative stability. It just takes more skill/effort/attention. The Wright Flyer had neutral (or was it negative?) stability. Certified aircraft are required to be stable in flight, although taildragger landing gear is inherently unstable on the ground. Go figure.

David