Yes, you can heat both sides of your tubs with flexwatt. You wouldn't need 2 thermostats, either. if you have proportional thermostat you can use a rheostat (dimmer) to cool down whichever side is too hot once you've got everything wired up.

And yes, enough flexwatt for 6 tubs, hot and cold side, SHOULD be ok for any thermostat on the market currently.