I took some time (10 min) today and hooked up a second switch so that I can choose to control one grate or both.
I set it up so that the second grate is hooked into one wire just like before. I interrupted the second wire with a switch so that I can choose to operate that one or not.
I was also out driving today and with temps at 55F I was holding 191F with both grates open. On my next trip I will see if only one grate open will keep the temps below where the fan comes on.