Yes, you're right. The smaller gear has to disengage. It must have been set up to unscrew (no lockring). But it would unscrew a lot quicker...

I wish I could find where I read about this. In the end, the guys figured out that the two gears needed to be very close in size, and that the friction of two drivetrains and all that screwing was slowing them down. Also there was a big plastic chain guide on the right seat stay that rubbed the chain. Not a speedy setup.