The diagonal legs need to move together.

In the Turtle bot, this is accomplished by sensors.

My initial idea was to use a fan gear and only run leg at one time on one side.

I had sensors built in to synchronize the action. 

Unfortunately, the sensors were limit switches that added load to the motion.  I could never get the motion synchronized.

"If only I could link the diagonal legs!" 

I tried to do it with a laser cut belt and cogs but it was not working well.

Then I came up with the idler concept seen now with the dual driveshaft concept.

This positively links the motion of the diagonal motors.

The timing to run each diagonal is determined by the half relative and half absolute

code wheel  that is on the shaft between the walls.  This runs through a ITR-9608, 

possibly the least expensive and most forgiving photo sensor in the world. 

The diagonals alternate taking steps for straight forward motion. Depending on the position of the inactive leg, moving the other diagonal continuously will turn in one direction or the other. This is the reason for the relative encoder with about a 10 degree resolution. I have not yet quite determined the best place to stop to turn each way.

Again,  I will drop in some pictures to show all these steps when I get a chance.