There is a table that maps products to categories so if you category 1 was face cream and category 2 was organics, the following sql statement would give you a list of products in both> select p2c1.products_id from products_to_categories p2c1
left join products_to_categories p2c2 on p2c1.products_id = p2c2.products_id
where p2c1.categories_id = 10 and p2c2.categories_id = 22However, that's a long way short of being a complete answer, as you would need to flesh that out to retrieve all the data that you wish to save, prepare it for display on a page, generate the HTML into which it will be inserted and finally integrate that with your Zen Cart.
In the current version of Zen Cart, it's a lot of programming, but there is a ton of example code that could be copied, depending upon where and how you wished to display these products.