With 1 Mosin on your back and one M4 in your hands select your canteen/food/other object. (your M4 should now be on the ground at your feet)
Attempt to pick the M4 up into your hands with the Mosin on your back. (you should only get the inspect option)
Drop your Mosin on the floor as well as the M4 and try to pick the M4 up (you will be able to)
Place the M4 on your back and try to take the Mosin into your hands. (you should only get the "Inspect" option
Relog with one weapon on your back/in hands and the other on the floor. When back in you should be able to take the weapon on the floor into your hands with the second on your back.