Can't buy one without the other.... only on your first purchase.
I am selling a product that has a programmable security key. You can buy unlimited numbers of the product, but you have to have at least one key. Theoretically I need to check if a customer has every bought a key before and make sure they have before selling them the product. If they have not bought a key before, then adding the product to their cart will either result in an error or both the product and the key will get added to the cart at the same time. If they have bought a key before, then they can buy whatever they like.
I could create 2 versions of the product, one with the key and one without and keep that in a "bought before" category and only show the product without to customers that have bought before, but that requires either changing their user group upon purchase of a key and restricting access to the "bought before" category, or some other such ACL logic.
There are probably a number of ways to do this, but I'd love to find an extension that handles this one way or another.