Sengir, the Dark Baron

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.

If Sengir, the Dark Baron is dealt lethal damage at the same time as other creatures, they die at the same time. The second ability won't resolve in time to save Sengir, the Dark Baron.

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 you and another player lose the game at the same time, the third ability won't resolve in time to save you.

Sengir, the Dark Baron's third ability will trigger no matter how the player lost the game: due to a state-based action (having 0 or less life, trying to draw a card from an empty library, having 10 or more poison counters), because of a spell or effect that states they lose the game, because your awesome might has left them with no choice other than concession, or for any other reason.