How to Add a Product to Multiple Categories
Every product on your store can be linked to multiple categories at the same time. While editing a product, you can set it's Primary Category in the Basic Details panel. Any number of Secondary Categories can be set via the Advanced Details panel - just click on the "Add Product to Additional Category" button.
While in the Manage > Products section, if you choose to Filter your results by Category, the Primary Category must be used to find any product that has been attached to multiple categories in this way.