Historical Rules · 5 October 2018
Rule 702.123d
Official source text from the 5 October 2018 Rules release
Except for determining the color identity of your commander, the two commanders function independently. When casting a commander with partner, ignore how many times your other commander has been cast. When determining whether a player has been dealt 21 or more combat damage by the same commander, consider damage from each of your two commanders separately. See rule 903.11a.