If two items are added to the cart during an upsell offer, it usually means that the theme is using an Ajax add to cart function. An Ajax add to cart function keeps the user on the product page when the add to cart button is clicked instead of directing them to the cart page. This functionality tends to bypass certain functionality of the app, and this is one of the issues it can cause.
One of two things can be done to fix this:
- Disable the Ajax Add to Cart Functionality in the Theme; or
Modify the Theme - If you would like to continue to use the Ajax cart along with Product Upsell you can Email our Support Wizards and we'll see what we can do to help you.