When you add a prints order to your cart, a prints project is created for you that is added to your projects page. If you try to add another prints order, the additional photos will be combined to the earlier print project, and you will see them all on the Prints Order screen and ultimately you will see only 1 project in your cart
Important: the paper finish that you first selected will be applied to any later sets of photos you add to your cart, so basically, if you want both matte and glossy prints, you must order and pay for them separately.