I wouldn't bother building a rack for two., unless you're really sure that you're going to have more. They can be space saving, though so if that's a consideration getting something set up would still be worth it.

And I don't use separate thermostats on my two tubs. I have the thermostat plugged into the wall and a powerbar plugged into the T-stat with the mats plugged into it. As long as the mats or heat tape are exactly the same that works fine.