We use standard household thermostats (the electronic style -- don't use ones with mercury switches) that have both heating and cooling settings, and these control relays which energize the roof air circuits. The thermostats have built-in five-minute cut-out delays so that the T-stat will not try to restart an AC until five minutes after it has been stopped.
We use 24vdc for control voltage and relays with 30-amp, 120vac rated contacts.
More complex then you need, but here is our circuit:http://odyssey.smugmug.com/Architecture/Drawings/63883_XJHit#2228266_dqfSH-O-LB
The part at the bottom with the yellow wire is the roof air circuit. You can see that we control three roof airs with only two thermostats, by diode steering. The middle air comes on if either the front or rear T-stat calls for cooling.
We just leave the built-in 'stats on the units on the highest setting.