I use two 20 quart (5 gallon) pots, can do 6 to 9 gallon makes. I usually bring to temp on the stove, then move the pots to a large cooler filled with warm water. I actually have an electric bucket heater (basically a plug-in heating element like those that go in a domestic electric hot water heater) for heating or maintaining the water bath temp in the cooler, and actually have a plug-in thermostat that I hook that up to for convenience in controlling the temperature. I occasionally have the two pots vary in pH by maybe 0.15 (due e.g. to slight differences in the amount of freeze-dried starter culture added), but nothing much to worry about IMO. I make sure to mix the curds together well in one pot after draining some or all of the whey (depending on type of make) so that any differences between the two are mitigated. The only shortfall with my setup that I have found, is that my water heater doesn't raise things fast enough to do a good "scald" on Alpine and hard Italian cheeses - it takes a little longer than it should, so I think I get a bit more acid than I should.