category JOIN question

Hi,

I’ve imported a big multiblog installation from Movable Type into WP with a hack so that the MT blog name would become a WP category. Thus there are now 4 primary categories that contain all posts of the respective previous MT blogs. And there are all the other categories. The problem I am facing is that some blogs are having some overlapping categories apart from the 4 main ones. So, assuming category 5 is “foo”, some posts from category 1 (first blog) and some from category 2 (second blog) have imported into the same “foo” category.

Is there a way to get, on, say, the “2” blog-category page a sidebar that links to a “foo” category that only contains links that are in both the category listed *and* the category whose page is being displayed?

In a way… show me all categories in category 2 to which there was also another category assigned. And then, if clicked, a category archive only containing those posts that are in both categories?

Is this possible using a standard WP function?
Or do I have to use a custom $WPDB->query to do that?
Or is it actually necessary to reassign categories hiearchically (so there would be two “foo” categories, one in the parent 1 and one in the parent 2).

Thanks for any help!

yms