Yes. For instance:
>Gun in right hand, nothing in left. You can use the Gun.
>>*Mouse3*
>>Gun goes to left hand, and nothing occupies right.
>Scroll through items menu. Items from scroll menu go through the right hand. Gun stays in right hand.
>Sword in right hand now. You can now use the sword, but not the gun.
>>*Mouse3*
>>Gun goes to right hand, sword goes to left.
>Can now use gun again.
Of course have limitations that don't allow the same item in each hand, duplicating items, etc.