Under the bulleted exceptions to Merging without review, there is a point that "A commit in Gerrit may have two authors" mentioning "an owner and a reviewer who uploads and amendment"; however, there is no exception for pair programming explicitly. I am not sure how common this is for MediaWiki core and Extensions (cf: scope?), but it is common for several repos I work in.
That is, a patch could be written during a hangout/pairing session between two people, uploaded by one, and merged by the same. As a recommendation, it could contain a
Co-Authored-By: snippet in the commit or two