Why would the AC system in my attic have 2 contiguous overflow traps leading to the same drip pan? Two AC guys have looked at it; one says it has to be replaced with a single trap because it is very likely to get clogged up, the other says it's 'probably' OK. Nobody can figure why it was done in the first place.

Where do you get your ac guys? the condenser pan is built with one drain higher up than the other. The lowest /main drain is the one that gets used all the time. If it was to get stopped up and not drain when the water gets higher it will drain out of the 2nd /highest drain port, therefore giving the water a second choice of a way to drain out, be glad it's there and plumbed correctly, A lot of installers don`t go to the trouble. I guess many installers don`t learn that the 2nd is necessary, and didn`t bother to teach the guys you had at your home

I like your explanation, but both traps are in the same PVC pipeline, only inches apart. Also, the pipe (port, I guess) has a cap on it and is not directly over the condenser pan, so I don't see how it can drain into it anyway. What do you think about that?

Any chance of a couple of pics? We could have more fun with it with that.