A small clarification when it comes to querying WordPress posts with multiple tags – tags in the URL are using Slugs …. not full values. If your query is missing items or is returning empty, make sure you are using the right value for your tags.
It is probably obvious to developers but it took me a bad attempt at multiple tags query to figure that one out.
http://server/wp/?tag=tags1+tag2 for a query of tag1 AND tag2 http://server/wp/?tag=tags1,tag2 for a query of tag1 OR tag2