You can use OR searches in the Search Within Results field or the main site search box, so
"Star Wars" OR "Harry Potter" OR Marvel OR "My Hero Academia"
in that field will get you all works with any of those four fandoms.
That's an elastic search string, not a tag filter, though, so you either have to go through Works Search - which doesn't let you use the sidebar filters - or start with another tag filter (like M/M) and then add the OR string to search within results at the bottom the sidebar.
The string I gave will also bring up works with those terms anywhere in the work, it's not searching fandom tags. If you want just the fandom tags you would have to find the fandom tag id #s and then use fandom_ids:[tag number] in Search Within and at that point it might be easier to just search each fandom separately.
(Also note this post is almost two years old now so some things in it have changed!)
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"Star Wars" OR "Harry Potter" OR Marvel OR "My Hero Academia"
in that field will get you all works with any of those four fandoms.
That's an elastic search string, not a tag filter, though, so you either have to go through Works Search - which doesn't let you use the sidebar filters - or start with another tag filter (like M/M) and then add the OR string to search within results at the bottom the sidebar.
The string I gave will also bring up works with those terms anywhere in the work, it's not searching fandom tags. If you want just the fandom tags you would have to find the fandom tag id #s and then use fandom_ids:[tag number] in Search Within and at that point it might be easier to just search each fandom separately.
(Also note this post is almost two years old now so some things in it have changed!)