Get them an ASP Baton. I hit a Pitbull in the head with mine, and he ran home. Two Pits were attacking my dog, at my feet. A walking stick would have been too long to be effective. The ASP was not expanded, so I used it in reverse. The mushroom buttcap was struck in the center of his head, and he instantly ran home.
Point is, a knife is a last resort. I'd want something with medium range on me, like an ASP, or a 15" long steel pipe, and a fixed blade.
With two people walking together, maybe one walking stick can be used if the other person is being attacked. Pit Bulls can be difficult to stop, they were bred to keep fighting. Each one is different, and may take more extreme measures to stop an attack.
I think a knife would have to puncture the lungs, or vital organs to get one to stop. That takes some strength, dexterity, and there will be lots of blood. The knife better have some grip.