Two pots put together will work. I put a big pot in a utility sink and fill the sink with hot water. That works great.
If you've cooked milk on a stove before you probably know how easy it is to scorch it. That's why you shouldn't heat the milk that way. If you stirred continuously, it might be OK, but even then the high heat at the base of the pot can damage the milk.
Any material is fine. Stainless steel is the best because it is so non-reactive. You probably want to avoid aluminum since the milk with be acidic after adding the culture (or acid if you're making mozzarella). I think acids react with aluminum, though I'm not really sure. But I do know that SS is perfect. Enameled pots work fine too. That's what I've been using.
