In order to have the same item in two different categories, you will have to duplicate the item and manually select a new category.
Once you select the duplicate button, a "Copy" of the item will appear and you can adjust any fields, including Item Category. The duplicated item will have all fields populated from the original item.
Tip: The duplicate tool is also helpful when creating similar items that share modifier groups and/or categories. Simply duplicate the item and update the Item Title, Description, and Price.