I'm late to this, but I am one of those "unacceptables" who carries a backup that is chambered in a smaller caliber than is my primary. To make it even more ghastly and shocking, the only spare magazine I carry is for the backup, not for the primary. This is so that, should more discretion be needed somewhere I go, the primary gun, worn on the belt, can be shed while leaving me my second gun (eight rounds in it) and a spare magazine (with an additional ten rounds.)
The calibers chosen are 9mm Luger and .32ACP. Both are Kel-Tec pistols. Sometimes, I even stick a third, even smaller gun, on board somewhere, a NAA Mini in .22LR, just because the new, engraved boot grips I just got for it are so cool.
Some of the people who insist that your backup is unacceptable if it's not as "capable" as your primary are less quick to castigate anyone who doesn't even carry a second gun.