This sort of adverting can be done by our software, but I do not recommend it. What is happening is that a small percentage of customers are getting confused and/or cannot add? As such it is causing arguments. A better solution would be either make a pack of two and sell the pack of two at a reduced price. This works well if you stick them so it looks and feels like a pack.
Another solution would be something like this by trying for three, so buy two and get one for free.
Having said this in general this form of marketing works well as if often you can sell someone something, it is much easier to sell them something else, and if they need one, quite possibly they will need two or more so you have an instant add on.
Furthermore, if you have a VIP system behind it with an email address, you can both offer them something else in a month's time when you send out a newsletter.