Partner is a keyword ability specifically used in the Commander format. A player can have two Commanders if both have Partner.
Partner was introduced in Commander 2016.
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 702.123. Partner
- 702.123a Partner is an ability that modifies the rules for deck construction in the Commander variant (see rule 903), and it functions before the game begins. Rather than a single legendary creature card, you may designate two legendary creature cards as your commander if each has partner.
- 702.123b Your deck must contain exactly 100 cards, including its two commanders. Both commanders begin the game in the command zone.
- 702.123c A rule or effect that refers to your commander’s color identity refers to the combined color identities of your two commanders. See rule 903.4.
- 702.123d 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.
- 702.123e If an effect refers to your commander while you have two commanders, it refers to either one. If an effect causes you to perform an action on your commander and it could affect both, you choose which it refers to at the time the effect is applied.
From the glossary of the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- A keyword ability that lets two legendary creatures be your commander in the Commander variant rather than one. See rule 702.123, “Partner,” and rule 903, “Commander.”
- Akiri, Line-Slinger -
Legendary Creature — Kor Soldier Ally
First strike, vigilance
Akiri, Line-Slinger gets +1/+0 for each artifact you control.
Partner (You can have two commanders if both have partner.)