I think you'd be able to get away with one t-stat. I'm assuming you'd tape or stick the uth directly to the bottom of each tub? If so, I'd drill a small hole in the tub, just big enough to fit the probe thru, then tape the probe right in the middle of the uth and seal the hole with some silicone. That way they're still stackable. You'll have to monitor the temp in the other tub closely to make sure they heat the same.