In the scope of one slot the replacement should be automatic.
So when you do something like "wield-primary sword", it will replace an item in the primary slot if (and only if) the wield command was successful.
What I mean, it shouldn't touch the other slot.

Another important thing:
A failed wield (the wield that would result inconsistent primary/offhand combination or can't be executed because of other reasons like cursed items or broken hands) sholdn't modify the current status of slots. It shouldn't remove or swap anything.