Kraum, Ludevic's Opus

• WWizards of the Coast2020-11-10
  Official Ruling
  An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.
• WWizards of the Coast2020-11-10
  Official Ruling
  One opponent must cast two spells for Kraum's triggered ability to trigger. Two spells from different opponents won't cause it to trigger.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  Kraum's triggered ability can trigger only once each turn for each opponent.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional 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.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.
• WWizards of the Coast2020-11-10
  Official Ruling
  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.

Rulings via Scryfall