I have ordered add-on items together with other items, and then shipped them to separate addresses (sending stuff to my kids away at school.) So while Amazon intended the add-on program to save them from having to ship small cheap items separately, they end up doing so anyway.
Recently I did this same thing, but then realized that I had shipped the main item to the wrong school address. So I cancelled that part of the order, and reordered it to the correct address.
As a result, I realized that the Add-on item ended up being in an order on it's own. I am not suggesting making and then cancelling orders as a way of circumventing the Add-on requirement. I am just observing that it may be possible.