Diesel's generate more heat than gas motors. More BTU's in the fuel.
Unless you're just talking low-speed light throttle stuff..
Personally, I'd follow the KISS principle.. wire them both together on a thermostat, with a manual override on a switch that's also tied into the A/C switch.
On mine, my toggle is on-off-on - the top position is thermo controlled.. middle is stream crossing, and bottom is manual on. Be sure to put a light on the dash that goes on when you shut the fans off.
(Terminal 87a is good for that)