So named because you get Four for the price of Two. This contraption is a gearbox that uses two motors. One motor changes the gears and the other provides drive to any combination of four outputs.

The idea is that not only do you get four output shafts, you can have any combination of them turning at one time, thus here are 16 different output combinations.

This was my second attempt at this concept. Trust me, you don’t want to see the first attempt - it was only slightly smaller than the Titanic!

This version is quite compact and as a ’proof of concept’ worked well. However to use it in a practical setup it really needs a way of realigning it at regular intervals as small errors due to flex and in particular the floating worm tend to accumulate until it gets out of sync and selects the wrong combination.

There are two output shafts on each side with a floating worm to select the required side. Then a rotor setup selects one of four possible combinations: none, output 1, output 2, and both output 1 and output 2.