Related: The Truth about knives
Most officers I see carry a backup knife in their pants pocket just under their duty weapon (strong side). I never understood this, since if I really needed the knife to get a suspect off of me and the knife is in my pants pocket on my strong side, why wouldn’t I just use my sidearm, since at that point you would be most likely be justified in shooting.
I carry my backup knife in my vest trauma plate pocket. I can get to it with either hand, and if I’m on my back in the full guard position trying to protect my face and neck area, the knife is right there for me to grab.