As I asked in an edit, you have a herpstat 2 right? You can't adequately regulate heat with only one control. A herpstat 1 will not cut it. You need to have independent heat control for each heat element, hence why a herpstat 2 is the minimum single thermostat that will work for this type of setup. If you have a herpstat 1, you need to get a seperate thermostat for the RHP or heat tape, which ever is not controlled by the herpstat.

Just want to make sure.