I think an AT-1 is a good backup for an A-1; they take the same lenses, use the same battery, they're the same size, feel the same. It's an all manual camera, no auto exposure, but familiar enough as to be easy to use, all the more so since the A-1 isn't particularly good at metered manual exposure control.
The A-1 would make a good backup camera to a T90, which is a fabulous camera, my favorite. Full automation, several metering options, built-in motor drive, great handling; very nearly an EOS 1V without the autofocus. And not terribly expensive these days, either. But, like the A-1, not so good at metered manual exposure control.
I've often wondered, though, what a backup camera should be. A replacement, in case one goes down? In which case the best backup would be another A-1. Or a camera that offers something the first doesn't have? The T90 has a higher degree of automation and better design, but none of the A or T series have mirror lockup, and if that's important then an F-1, F-1n or FTb might be a better choice.