A simple solution might be to make a symlink of the resource under the other category you need a product under.
If the store model gets complicated, this might not be the best option in terms of managing your products.
You could look at either a multi-select list item, or tags, as a tv method to put them in different categories.
Again, there are shortcomings to these methods depending on how many categories/products you have. At that stage, you should probably be custom writing snippets, perhaps even having custom tables for products.