I usually carry a .38 S&W hammerless J frame 422 as a backup in a DeSantis ankle holster.
I keep it fed with .38 +P 124 grain Winchester semi-jacketed hollow points.
My main carry gun changes from time to time, but I now carry a stainless Walther PPK/S .380 as a carry gun. I sometimes carry a Browning Hi-power 9mm as a main carry gun.
If anyone is involved in a shooting, and the cops try to think of you as psycho for carrying two guns, explain the following:
1) Main gun can jam, or you can run out of ammo.
2) A backup gun can be passed to an unarmed ally.
3) A criminal can take your main gun away.
And so on.
When I walk around armed, I try to have answers to questions ready before they're asked.