Thread:Extension talk:Semantic MediaWiki/Reunion instead of intersection/reply

The documentation for this is at https://semantic-mediawiki.org/wiki/Help:Selecting_pages (see "Unions of query results..."), where you'll find that you can do so in two possible ways.

FWIW, there's also Extension:Semantic Compound Queries, which does something similar although it is in some ways inferior to using #ask.