I dive solo and I always dive with 2 knives. One i wear on my belt harness in DIR fashion. This knife is small and is not that easy yo get at, so it is my back up knife. I wear another on my leg which I use all the time. I got wrapped up in line once on a solo wreck dive and then dropped my only knife. i vowed that if i got out of that pickle, i would adopt the DIr config and carry a second knife.
The only thing I changed on the DIR waist strap configuration is an elastic lanyard that locks the blade into the sheath and prevents accidental loss. I think that DIR says not to add this improvement, but since it is a back up knife, I'm not worried if it takes an extra second to deploy it.
adding an extra knife back up knife has very little drawback and if you coat the blade with vaseline, it will not rust quickly, even if you barely rinse it.