A player holds two independent Sports Axiom Levels: one for singles and one for doubles. Singles results only ever move the singles rating; doubles results only ever move the doubles rating.
Why separate?
Court coverage, shot selection, and rhythm differ enough between the two disciplines that one number would misrepresent many players. A strong doubles specialist is not automatically the same level in singles, and the ratings should not pretend otherwise.
Doubles specifics
In a doubles result, both members of each pair get an update to their doubles rating, weighted against the opposing pair's combined level. Each player's Confidence Score is tracked per discipline too, so a frequent singles player entering their first doubles division starts doubles as provisional.