If your Commander deck has two commanders, you can only include cards whose own color identities are also found in your commanders' combined color identities. If Falthis and Kediss are your commanders, your deck may contain cards with black and/or red in their color identity, but not cards with green, white, or blue.
If something refers to your commander while you have two commanders, it refers to one of them of your choice. If you are instructed to perform an action on your commander (e.g. put it from the command zone into your hand due to Command Beacon), you choose one of your commanders at the time the effect happens.
Akroma's ability refers only to creatures with partner or "partner with" abilities, not creatures with any other partner variant (such as choose a Background or friends forever).
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Chris Rahn
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To have two commanders, both must have the partner ability as the game begins. Losing the ability during the game doesn't cause either to cease to be your commander.
A creature with more than one of the listed keyword gets +1/+1 for each of those keywords it has.
Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional {2} the first time you cast the other. A player loses the game after having been dealt 21 damage from any one of them, not from both of them combined.
A creature with more than one instances of one of the listed keywords gets just +1/+1 for that keyword. Similarly, if a creature has multiple variants of the same keyword (e.g. protection from black and from red), it gets just +1/+1 for that keyword.
An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.
You can choose two commanders with partner that are the same color or colors. In Commander Draft, you can even choose two of the same commander with partner if you drafted them. If you do this, make sure you keep the number of times you've cast each from the command zone clear for "commander tax" purposes.
Which creatures are affected and the size of their individual bonuses is determined only as Akroma's triggered ability resolves. Gaining or losing keywords after that time won't cause a creature to grow or shrink.
Both commanders start in the command zone, and the remaining 98 cards (or 58 cards in a Commander Draft game) of your deck are shuffled to become your library.
