This could be remedied by having the primary weapon in hands go directly to the holstered position if the player switches to a holstered primary weapon using the number hotkeys. To function in the inventory screen, a drop-down menu with an option to 'swap M4A1 (or whatever 2-handed weapon) for SKS Rifle (or whatever 2-handed weapon)' when you drag and drop the holstered weapon onto the weapon in hands (or vice versa) would allow to switch the weapons without number hotkeys. Currently, the fill of the box around the 2-handed weapon turns red (as if one of the two inventory spaces cannot accommodate the other) when you drag and hover a holstered primary weapon over a primary weapon in hands (or vice versa).