This shows the assembly; in the original design I had the idea of having an 'L' shaped unit but it proved to be too unstable when stepping around.

Directly attaching the motors together was the best option, although I still needed a counterweight to stop the circuit board from the falling down because of the vibration.

Yes, that is lego you see! It's actually part of the Lego Mindstorms kit, which is a gentler (if more expensive) introduction into physical computing using a microcontroller.