The bottom potentiometer is used to set the center point, the top would simulate the movement of the sun. This might be used for many things where you want to control the direction of a motor with respect to another value.
I have bread boarded this and it works. The two resistors on either side of the top pot were replaced with CDS cells. When they receive equal light the circuit is balanced, when one is shaded the circuit is unbalanced and polarity is shifted either positive our reversed. On my solar tracker I found it helped to put the CDS cells close together with a divider between them.
You will not get positive and negative, rather positive and less positive. This time I removed some things that were not needed. The biggest limiting factor in the application of this circuit has been the capacity of the transistors.
