The Herpstat probe should be on the outside of the tank, between the UTH and tank. The thermometer probe that only tells you temperature should be on the inside of the tank, under the substrate, above the UTH...
I have tubs set up, but it is still the same for tanks. Is this how you have it currently?

1) The Thermostat (Herpstat)probe should be mounted something like this to the bottom of the enclosure...



2) Then the UTH is mounted to the bottom...



3) The thermometer probe is buried in the substrate on the right hand side and does not affect whether or not the Herpstat is running. I only tells you what the current floor temp is above the UTH on the inside of the enclosure...