In theory that is correct. I was somewhat concerned about how it would work in practice, in view of wear and mfg differences in the motors.  I did not run the planter in question this year, however when I ran it right after putting it together, the monitor was not able to detect any difference between the population on each section.

Yes, the "extra" motor runs nothing but the speed sensor.  That was the only (well, simplest, anyway) way I could come up with to be able to run the planter with ANY section shut off.   It was originally set up so that the center section had to run any time any section was running, but I did not find that to be acceptible. 

I spent some time running behind the machine this season while adjusting the delays on the section controls, and I was not able to percieve any difference in rpm between the ends of the different sections of the drive shaft, which in some cases are end to end within an inch of each other.