It seems that search/advanced has the same problem as in "The tagged parameter does not work with multiple tags" 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%23;.net&site=stackoverflow
tagged- a semicolon delimited list of tags, of which at least one will be present on all returned questions.".