Tevesh Szat, Doom of Fools

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.

For the second ability of Tevesh Szat, Doom of Fools, you choose whether to sacrifice anything and what to sacrifice as the ability resolves.

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.

As the third ability is resolving, you'll gain control of all commanders that were already on the battlefield before the commanders from the command zone enter the battlefield under your control.

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.