Well, my first choice would be a matched pair of H&K P7M8's, but since you stipulate a mismatched set, I did used to carry a Sig 220 as primary and a 225 as backup and I was very comfortable with that setup. A pair of Glocks would also work, or a 45 and a compact, most manufacturers can supply such a set.
I do go through phases though, for a while I carried a pair of Glock 17's with +2 baseplates, 40 rounds of 9mm+P+ without reloading! That was a little much to carry, so I went on to the Sig set and never looked back. For a short while I lugged a 4" S&W 686 and a Bodyguard as backup, that setup also worked well, but was real heavy.
I do think the manual of arms needs to be similar between the two pieces, or at least the backup needs to be a double action of some sort, preferably without a manual safety.
My reasoning here is that by the time you get to the backup, one of the following has happened:1. You ran the primary dry and you're in big trouble. 2. Your primary jammed and you're in big trouble. 3. You lost your primary and you're in big trouble. 4. You need a piece to lend to someone because they are unarmed and you're both in big trouble. You can see where this is going.
In any of these instances, the backup piece needs to be simple to operate and reliable, which, IMHO, is also a good criteria for a primary piece. Just my $.02, for what it's worth. Hope this helps.
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With my shield or on it...