Air Conditioning - Control of 2 A/C heat pumps that share a duct

Our new (to us) 30 year old house has two A/C Heat Pump units that share a duct system (either can heat/cool, or both can. In the summer, both run at the same time to cool 3600 sqft, the return duct tees two two filters and runs through two evaporators to the main in-floor duct system. A pressure valve acts as a check valve to prevent backflow if only one unit is running). Each unit has its own controller, set about 8 feet apart in the hall, so they are essentially measuring the same temp. They are the old style mercury switch, and I would like to replace the two of them with one single controller (programmable). Has anyone seen a system like this, and if so, how would I go about uniting the controllers? Or is this a really bad idea?

Thanks for any help!

It can be done. Are you sure that you want both units running together? What if you dont need as much cooling or heating, wouldnt you rather have them controlled seperately?

Do both units cool and heat the same area? If so, you could add some controls with one thermostat to stage them, that way the first unit would start and if and when it cant keep up, the second unit would kick in. I think this would be the best way to set it up. You have kind of a wierd system there, dont see that too often.
I usually build my own controls but there are devices out there that you could have installed to do that.

Well, I guess that ideally one would start, and if it was not keeping up, the other would kick in. They both cool/heat the whole house. In another house (we are Air Force and so move a lot) we had a 2 stage that worked like that, but it was a single unit. Perhaps two programmable controllers is the simpler way to go. Yes, you are right that it is a weird setup. It took me a while to figure it out. There are lots of other oddities, including a defunct solar radiant heating system that I may convert to a water heating system.

Well, I guess that ideally one would start, and if it was not keeping up, the other would kick in. They both cool/heat the whole house. In another house (we are Air Force and so move a lot) we had a 2 stage that worked like that, but it was a single unit. Perhaps two programmable controllers is the simpler way to go. Yes, you are right that it is a weird setup. It took me a while to figure it out. There are lots of other oddities, including a defunct solar radiant heating system that I may convert to a water heating system.

I guess if you didnt want to add some controls, you could have two equally calibrated thermostats and just run them a degree or two apart from eachother.