I tried to convince Matthi before posting that two hand slots are very useful and wanted to prove that the concept is very handy. But the only two item combinations I could come up with were "melee weapon & shield" and "bow & jar of winds". The jar of winds may give rise to other combos (throw flint + boost it) but I can't say that this is superior to a system with 1 slot and a quick changing mechanic.

I really like the interaction bar, too. So personally I would prefer to keep it intact. To change your selected item we can rebuild the backpack menu to a more quicker access: Hold down q (menu closes on release) and move the mouse nearest you the item you want to select. That's more or less like Matthi's direct access. If you have time to do stuff, you can drag'n'drop items here to rearrange your backpack.
Obvious next thing would be the mousewheel but it's very imprecise.
Or shift + number key.

A counterproposal to interacting with rightclick would be to have 2 item uses on the two mouse buttons. Like for the axe it's chopping with left click and attacking with right click. The sword has attack and block. The shield has block and shove. And so on...