hey charismabiz,
i had the same need as you and i managed to show only posts that were under two categories. i adapted the code from the following:
https://codex.www.ads-software.com/Template_Tags/query_posts#Excluding_Multiple_Categories
it’s kinda dumb solution: i first queried one category with query_posts() and then narrowed to the posts that also were under the second category with the conditional in_category() placed inside the loop.
doing this way, wp will only show the overlapping subgroup of posts that are under both categories ??
