Historical Rules · 20 November 2020
Rule 702.123d
Official source text from the 20 November 2020 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).