You set the temp on the chiller for a 1 - 2 degrees higher than what you set the Apex to. The Apex will manage that temp very accurately, turning the chiller on/off as needed. If the Apex ever fails (assuming you have Fallback ON) then your temp will rise that 1 - 2 degrees and the chiller's t-stat will take over, protecting your tank.
So as an example, if I want my tank temp to not go above 79, I would set the Apex to 79 and my chiller to 81. The Apex will cycle the power to it before the chiller tries to turn itself on.