The idea would be to place the probe into just one carboy, connecting the two Fermwraps into a powerstrip to the Ranco. Since the 2 carboys would be very close in total volume, same material, etc. the temperatures would be fairly close and therefore 2 Rancos would be overkill. Whaddaya think?
If both beers are identical it would probably work okay. They would need the same gravity, level of disolved O2 and pitch rates. Or at least very close.
I have two beers in my ferm chamber now, one a barley wine and one a low gravity ale. I pitched a yeast cake in the barley wine and a single tube of yeast in the basil ale. now, after three days they are the same temp but for the first two days or so the BW was almost 10 degrees warmer than the basil ale. I ‘control’ my temps with a cheap-o timer but had I had a single controler either the BW would be too warm (if I was monitoring the basil) or the basil would have been to cool (if i was monitoring the BW)
I would be concerned using a fermwrap directly on a glass carboy. I don’t think glass would respond very well to the rapid changes in temperature that would come when the fermwrap kicks on. If they are plastic carboys, then no worries.
Mine Fermwrap is 4 years old and I have used it on glass and better bottles.
Fermwrap is great at holding temps but will only heat around 7 degress higher than ambient temps. (So for a saison - it will not take you from 64 to 80F)
I don’t think you’d have a problem using one ranco to control both ferm wraps. especially if you’re doing 10 gallons split into 2 carboys then you’re looking at the same og and same starting temp. placed side by side you should have near identical temps.