Did you ever try this? Basically I am wondering if I can have one ale fermenting at 64*F and the other cold crashing to 40*F in the same fermentation fridge.
Don't know if davidhenry figured this out but I've been using the fermo-temp/ fermowrap as of late to control the fermentation temp with the coming cold temperatures.
Fermenting an ale at 68-70 degrees F along side a lager in a chest freezer at 40 degrees F is very possible. The major difficulty would be getting an accurate temperature reading for the temp controller that the heating wrap is plugged into. If you use electrical tape to apply the temperature probe to the side of the fermenter, chances are it will pick up the temp of the outside of the fermenter which chances are is not the same as the center of the fermenter. Thus the temp control will keep the heating pad on or off for too long and you won't get the correct temperature you need.
Luckily I have an SS brewbucket with a thermowell/ diptube on the lid. It's a hollow tube that, depending on how much beer is in your fermenter, is submerged in the middle of your fermenting beer. I put the temp control probe into the dip tube so that the unit gets an accurate read on the overall temperature of the beer and not temperature of the fermenter.
Now I really had this problem with my plastic Speidel fermenters. I couldn't get an accurate temp reading because I taped the temp probe to the side of the fermenter. For those of you who use carboys, plastic or glass, this will also be a problem. In doing some research, on Amazon I found a standard rubber stopper with a diptube attachment. The rubber stopper is a standard size, has a long dip tube for the temp probe to go into and has another hole for the standard airlock/ blow off tube. This has solved my problem for innacurate temperature readings for my temp control. You may need to verify the width of your carboy/ fermenter mouth or hole for the rubber stopper. It's a #7 stopped and fits nicely in my Speidel fermenter. I'll post the link at the bottom.
Accurate temperature readings is the main concern here. The only other thing I can think of is that your heating pad will most likely be on at all times in a 40 degree fridge. Depending on what your temp differential is set on your temp control, once the fermenter hits 70 degrees F and the heating wraps shuts off, it won't be long until it drops and the heating wrap turns back on. This may effect the life span of the heating wrap and use more electricity in the long run.
A small investment of about $15-$20 for the thermowell/ diptube will make a very large difference in the temp control, your fermenting capacity and your space efficiency. Hope this helps out a bit.
https://www.amazon.com/gp/aw/d/B01KP4D0K2/ref=mp_s_a_1_7?ie=UTF8&qid=1484009095&sr=8-7&pi=AC_SX236_SY340_QL65&keywords=fermentation+heater&dpPl=1&dpID=21vesuZ7bIL&ref=plSrch
Cheers!
