where is this house located? Sounds like you had a commercial AC guy who lived in the house or an engineer that wanted to think he's cool because he does something different.
You actually need a pro to come out and take a look at this system. It's so hard to say what you have with out looking at it.

The inspector was not surprised because he doesn't know.
sorry to say the majority of inspectors only follow code and not application scenario's.

K,
let's go back, let's talk commercial applications
Do mean it has 2 condensing units that have a compressor in each that is going to one evaporator coil?

Does it have 2 different line sets going to the evaporator coil. Meaning 2 suction and 2 liquid lines. How is the line sets ran?

The way you have described it is a lead lag compressor set up but then you throw in a 4 ton coil that screws it all up with a lead lag scenario.

There would have to be a 3 way solenoid valves on each line.
and to be honest this set up is really to much to write out with out looking at your situation

My advice is to change it out. If you are submitting a bid. Summit it with the aspect of changing out the A/C system.

That system is going to give you a headache, it's old, it's R22 and oviously there is something up with the way it was designed. There are advancements now that can do better what that system is attempting to do.

Thanks, classicdave, the house is in the New Orleans area. It is an 80-yo, 2000 sq foot frame, raised building. There are additions to the original house structure.

The guy that owned the house was an electrician. I prefer to think that the inspector was being truthful when he said that he had seen compressor arrangements like this before. I wish I had been able to take more time to look at it with him. I had video plumbing and termite inspectors there at the same time so I was pretty occupied. He had plenty of opportunities to shut me down, but took plenty of time to explain things as we crawled under the house and around in the attic together after the other guys left. When I said he was not surprised, I meant that he said that he had seen it before.

I will have the opportunity to ask him follow-up questions. I hope that you can help me make the most of it. We have to make a decision and seal the deal quickly to make the closing deadline that we need to meet. I hope that we can get an AC professional to see it.

Yes, there are two compressors driving one evaporator. Unfortunately we were looking at the outside stuff when the plumbing guy came over to speak to me so I did not get a great chance to look at the AC plumbing. I think that, outside, there was nothing to note, but the two line sets. I cannot say where they went together. I can ask the inspector on Monday. When he was looking at the evaporator and air handler, I was looking at the termite damage on some rafters.

Please explain why one evaporator wills screw-up a lead-lag set-up. Will one compressor never get the evaporator to a low enough temp to control humidity? If that is a yes, then two evaporators would be the usual way to do this, correct?

Way back in the OLD days, several manufacturers made dual circuited coils for residential sizes. Common on the big stuff, probably couldn't find it now for the small stuff. The coil was set up for a 2 compressor outdoor unit or dual outdoor units. Fedders & Westinghouse did this for sure. So did Trane (before they bought GE). The system was run by a 2 stage thermostat just like today's high end 2 stage stuff. Probably no lead-lag, same one was always first stage.

Got any more info? Model #s?

Thanks for your response baldloonie. The first compressor is BDP model number 568B024RCUHL (SN 4879AD0193). The second compressor is American Standard 7A0024H100A1 (SN N1641LBAF). The furnace is Ducane model FPBB100A4 (SN 99156D9850).

I see what you mean about dual circuit coils. I guess you could get to nearly the same place by installing two coils. I have a question that has arisen in my mind. What kind of evaporator does a modern two-stage system use?

A modern system has 1 suction line and one discharge line going into the evaporator coil and they use a 2 stage compressor(copeland or Bristol) or like trane a large size compressor and a small size compressor. In your case the trane XL20i would have a 4 ton compressor and a 2 ton compressor.

an older style coil like the one you may have has
2 suction lines and 2 discharge lines.
In low stage one of your compressors come on and it supplies half of the coil capacity. High stage both line sets are being used.

Once this coil leaks you will have to replace the system and I can bet the coil is not new. I would strongly suggest getting the A/C replaced with the deal and putting it in the 30yr mortgage.

Your situation, you mentioned only one compressor ran at a time. However, if they are both 2 ton compressors one would run then the other would come on at high stage and they would both run at the same time.

It seems like we have a couple of people that are familiar with this kind of arrangement. That means I can ask again, how is the second compressor usually controlled? I guess the first one is controlled by the thermostat, but how does the second one "know" how to turn on?

I really NEED to understand how this works or it will drive me nuts!

DO you have 2 complete linesets entering the indoor coil? That would suggest a dual circuited coil which is the best way for such an arrangement. Otherwise they could have somehow tied both outdoor units into 1 lineset and conventional coil.

The Bryant is a deluxe unit, 30 years old, good life.

Most likely they have a 2 stage thermostat that senses that the one unit isn't keeping up and brings on the 2nd to help it.

I just got some evidence tonight that it is a split condenser. The mystery is solved, I think.

Thanks, everyone, for your contributions. There is a leak in the condenser so I may have some tough decisions to make. I hope you might help me out in the future.