The docs for search/advanced (https://api.stackexchange.com/docs/advanced-search) have this to state about the
"tagged - a semicolon delimited list of tags, of which at least one will be present on all returned questions."
I read that to imply that it's an OR search. However in my testing, it appears to be AND. So for example:
Returns questions marked with both tags - but maybe that's luck. But if I add a third tag:
It returns nothing.