The herpstat two has two outputs. Think of it like two seperate thermostats. One probe tells the thermostat what the temperature is where ever the probe is. So if you have it on a heating pad, it will tell your thermostat what the heat pad heat is.

The other probe is for another heat source. If you are only using one heating pad then you will only need the one probe, if you are using two heat sources, then you will need the other probe.

An example, I have a herpstat pro, which has four outputs. During breeding season, I ran a female rack system on one output, a male rack system on one output, an incubator on one output, and lighting on the last output. Each output had its own probe.

I hope that made sense.