So what I am asking for is a mod that allows you to combine 2 items in your inventory, makes an item pair, then you can take the apart back to two separate items. If you were to make a GUI for it for more user friendly access I made a rough draft of what it should be like

Now if you were to take another path by making this accessible through /commands you can do
/combine (item1) with (item2)
/take apart (itemcombo#)
if you want to make it specific hands then it could be
/combine R(Item 1) L(Item2)
I am sure this is possible to do and would be great to have.