I have some desire, but it would take a long time to figure out no doubt--AGREED!
Some people familiar with GHL programming says there is no way to do this and i should submit this to GHL's "wish list".
Having chiller and heaters run off their own thermostats, having GHL shut off either if one malfunctioned, and preventing heaters and chillers from running at the same time seems like pretty basic function of an aquarium controller.