It seems that `search/advanced` has the same problem as in "[The tagged parameter does not work with multiple tags][1]" question.

When I select `c#; .net` I only get results tagged both `c#` and `.net`:
http://api.stackexchange.com/2.1/search/advanced?order=desc&sort=creation&tagged=c#;%20.net&site=stackoverflow

  [1]: http://stackapps.com/questions/652/the-tagged-parameter-does-not-work-with-multiple-tags