Gun A in your back, Gun B in your hands
Open inventory slide Gun A to ground (remove it from inventory)
Put Gun B away by clicking on the red cross top right of "hands" section (this will make gun B appear in inventory on your back
Gun A is still visible on ground, right click on it and choose "take Gun A"
Gun A and B are in your back (one at top on inventory as usual, one right down at the bottom). Your hands are free, to carry another gun, or anything else, or just free so you can run fast etc.