If you go to the bottom right hand corner of Calibre, click on the word "Layout", then show the "Search Bar" which will appear below the top row of buttons.
You can then click on multiple tags in the left panel and the code shows up in the search bar. You can then check the way the search code is structured. You can include and exclude tags in/from the search.
Code:
tags:"=Fantasy" and tags:"=Fiction"