if the heat tape is the same length, size and wattage for both tubs you can wire them together and use the same two thermostats you have now. Either daisy chain the two hot side tapes together and plug in to one thermostat or wire separately and plug in to a strip and then the strip in to the stat. Do the same for the other side tapes and your running both tubs off the same two stats. That's how you would do it in a rack so it would be the same for just two tubs. If the flexwatt is different then you'd have to either get more stats or switch the tape so they are the same