I can't picture how that would work without a proliferation of select boxes, so I checked a number of web shops that I consider to be exceptionally well implemented and none of them has anything like this. What they DO have is a link on the cart page which will take you back to the product page for the product you just added. That makes it easy to add another with a different size, color, etc. It might be a good from a usability viewpoint for UC to add a link like this under the "Continue shopping" link ...