Backup to my Kel-Tec PF9 was, for over a decade, a P32. Last year, I switched the belt gun for a Taurus G2C, and the backup for a Ruger LCP-MAX. These are my first carried striker-fireds, and were both first tested on the same day.
I've been wanting to one day ditch the belt gun altogether, which would make the LCP-MAX become the primary. Its backup would be a NAA Mini in .22LR, which I have taken to carrying in my jeans watch pocket, butt to left (I'm left-handed), hidden behind a cell phone pouch.
It's also possible I might be able to make the P32 work as a backup to the MAX. I didn't get rid of either of the Kel-Tecs.