by Eric Strom.
There is an old plugin supporting 2.1
https://docs.moodle.org/21/en/Multiple_Markers
which enabled 2 graders (but unfortunately not more than 2) for assessment. Maybe someone would be willing to take this up for further development.
It would be a wonderful feature request to allow averaging (and even other calculations) of assessments from multiple instructors for the same submission. Applications include adjudication by committee and team-taught classes.