LIst. A mere scrap of paper we keep nearby on which my spouse and I jot down whatever we see is or will soon be depleted and need to be renewed. The first of us that is able to do errands simply grab the list and that's it. The moment we notice the list is not where it is supposed to be 1) we know the other is doing errands 2) we start a new list ;)

Doing so, we never have to wonder what we need to buy when we're doing errands, we just follow the list.