It is much easier to carry two 8-kilo suitcases then one 10-kilo suitcase.
Is it an obvious paradox? No, it is not, because when two hands are loaded, the muscles experience a uniform load. If the load is concentrated on one side of the body, the load on the muscles is much greater. Part of the muscles holds the load, and the rest of them must ensure equilibrium while walking.
In general, it would be better to carry a heavy load in a kind of a post-bag hanging on a shoulder, or in a backpack. A backpack is an excellent and ergonomic alternative for a shoulder bag, which only provides load on one shoulder. Whereas a backpack distributes load symmetrically between both shoulders and, in addition, partially distributes pressure over the back.
However, a backpack is not ideal either. A heavy backpack weights down shoulders, changes the normal center of gravity of a body, spoils posture and makes one stoop.
The inventor Shimanov found a simple way out. He additionally divided a backpack into two parts (patent application 9127671, Russia). One part is fixed on the back as an ordinary backpack, while the other one is fixed on the breast. The load is distributed so that it allows an upright posture to be preserved.
Trully, full-breasted women will hardly like this invention.