I got it wired up like so (and working beautifully):
One thermostat controls everything. Regular heat pump system with electric coils as aux/backup heat.
I tapped into the Y1 (yellow) wire that controls the compressor to the outside unit. I have an aquastat strapped to my supply manifold that senses water temperature. When the water temp is 120 degrees or better, the aquastat breaks the Y1 wire so it is not complete (and therefore, the compressor will not come on). When the thermostat calls for heat, the air handler kicks on and and a zone valve to my coil in the plenum also opens via a relay. If the thermostat needs more than 4 degrees of heat, the AUX heat kicks on. If water temps in the supply manifold are below 120 degrees, then the aquastat completes the Y1 wire and allows the heat pump to function as normal and the zone valve closes so my heat pump doesn't heat the water in my OWB.