Kraum, Ludevic's Opus

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.

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.

Kraum's triggered ability can trigger only once each turn for each opponent.

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.

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.

Kraum's triggered ability will resolve regardless of whether the first or second spell that a given opponent cast that turn has resolved, was countered, or is still on the stack. Notably, the ability will always resolve before the second spell resolves.

Kraum's ability looks at the entire turn to determine which spell is a player's second spell. It doesn't matter whether Kraum was on the battlefield when the first spell was cast.