Thanks for pointing this out. The problem was worse than that, I’m afraid. I’m sure that what I had in there worked at some point, but it was not very smart about different URL settings on different sites, and I was finding most recently that the query string
?cat=slug1+slug2
was not pulling up the intersections of the categories specified. It wasn’t anything you were doing, at any rate.
I’ve just posted a new release that tries to handle the business with more grace. I detect whether a site is using permalinks (pretty URL’s). If so I try to use that URL structure to formulate the URL appropriately for the site and then direct the page to that location. The idea is to keep the intersection links consistent with the other links on a site.
When a site is not using rewritten permalinks, I leave the URL with the query string as it is generated from the widget, but I then use those values to run a custom query to pull up the appropriate posts.
I foresee that additional tweaks will be required, but I hope that this is a good start.
