There are two reasons to have two cam straps. The usual reason is it makes your BC look cooler and is mainly a marketing ploy. The only real reason is if the backplate isn't curved to fit a tank, the tank will twist on the BC. Most backplates on jacket style BC are curved to hold a tank (see that hard piece of plastic in your BC where the cam strap attches, that's the backplate). On setups such as traditional backplate and wings, there is no curve in the plate so two cam straps must be used.
If your tank is slipping, you may not have the cam strap theaded through the buckle correctly (they are not theaded in an intuitive way) or the strap isn't tight enough or you aren't wetting your BC and the expanding strap is causing you to drop the tank.