Historical Rules · 23 July 2021
Rule 702.124d
Official source text from the 23 July 2021 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 (see rule 903.6). 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 903.10a).