A natural extension of the useless machine, it's just a small logical step to put 2 arms inside the same box. There is a place between the two 'on' positions of a toggle switch where neither side is actually on. From this precarious position even a little bump can cause the switch to fall into one of the on positions, leaving the arms to battle it out forever.
It's very similar in principle to the other machine, all simple electronics and wiring. The toggle switch crosses over the battery wires to change the direction of the motors. Each arm only moves forward when the opposite limit switch pushed. (svg here)
These machines are available as kits through my shop
41 Projects / 47 Weeks