This is the part that you have to make a leap of faith with:
You set the temperature for the rear with the driver's-side temperature flip-up, flip-down switch. You can only do that when you're effectively in the rear seat climate mode, meaning that you've pressed the RR button. If I remember correctly, it's time that gets you out of that mode. So, so many seconds without activity, probably 15 seconds. Then the driver's-side temperature adjustment will apply to the driver again.
By the way, it's possible that the passenger-side temperature control works for this purpose too. I've just never tried that. It should do it if it doesn't.
2017 sport (just went out to the truck to investigate) .....
- RR settings button takes you from rear settings to drivers settings (back and forth, no wait time)
-the passenger can adjust the RR temperature