WordPress does this automatically if you do
http://yoursite.com/?cat=1+2 (where 1 and 2 is category ids)
OR
http://yoursite.com/?category_name=apples+oranges (where apples and oranges is the slugs)
OR something like
WordPress does this automatically if you do
http://yoursite.com/?cat=1+2 (where 1 and 2 is category ids)
OR
http://yoursite.com/?category_name=apples+oranges (where apples and oranges is the slugs)
OR something like