We use an old refrigerator with a temp controller for our fermentation chamber. Saturday we brewed two 5 gallon batches of different beers with the same desired fermentation temperatures. With a single batch, I'll tape the temp probe to the fermentor and insulate it. With the two fermentors, I taped the probe to Better Bottle #1 and positioned better bottle #2 so that the temp probe was sandwiched between the two fermentors. I didn't use any insulation. I know that internal fermentation temperatures vary and that my solution is just a compromise. So far it's working well (temperatures within a degree or two but I'm wondering what others have done.
BTW - The back to back brews (with overlap where possible) kept us busy for 8 hours but ultimately saved us about 3 hours off the time for seperate brew days. My wife picked and brewed one recipe and I did the other. We'll do it again.