You'll have to figure out where to set the heater, as its internal thermostat won't be calibrated to the thermostat in the chiller's temp controller. You'll want to use a thermometer you trust and compare it to the chiller's readout get your water to 72*F if that's what you want your lower limit to be (you heater should be truned "off"/unplugged, but still in the return loop). Once you do that, you'll want to slowly turn your heater up until it just turns on. Now back it down until it just turns off. This will insure that your water never drops much below 72*F.
Of course, the simplest way to do this is to run both pieces of equipment from an external dual-temp controller like a Ranco. You set the chiller to 70*F, the heater to 80*F, plug them both in and set the controller to 74*F with a 1*F or 2*F differential.
Once you have it set up, you'll want to keep an eye on both pieces of equipment to be sure they aren't active simultaneously, as they're working against each other if this is the case.
