I often find myself carrying a backpack along with another bag (or 2) and a camp chair, etc, etc, but I have sloping shoulders and those additional items often slide off my shoulders.
On my camera bags, I have a Peak Design Capture Pro on the straps to hold cameras,
but I also found them handy for keeping other bags from sliding off my shoulders. You can do this, too, with something as simple as a bandana.
Just double-knot a bandana on the strap and slide to where it help you the most. You can tuck the extra material under the knot, but I found those popping out pretty easily.
Tada! and happy schlepping!