I'd recommend the Herpstat or Helix proportional thermostats. Proportional means that it will regulate the amount of current to the UTHs so they normally won't be turned on completely, but rather slowly like a dimmer. The UTHs usually get around 110-120F, but they are known to malfunction if left unregulated and can potentially burn the tub. You can place the UTHs directly on the tub with a thermostat or dimmer so there's no need to leave a gap more than the 1/8" gap or so that is already underneath the tub.

The tub you selected looks fine, and you can either use clips to secure the sides or a removable zip-tie solution. The snakes should be okay without it, but the added security is nice.

Oh, the thermostats usually come with one probe (they make more expensive ones with two or four probes, but those aren't necessary in your situation). If the tubs and setups are identical, then just place the probe in one tub and plug both UTHs into a power strip that is plugged into the thermostat. The thermostat will regulate both of them equally.

Let me know if you have any more questions.