Dargo, the Shipwrecker

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.

The cost reduction is {2} per sacrificed permanent, whether that permanent was an artifact, a creature, or both.

Dargo's first ability can't reduce its cost below {R}. You can sacrifice any number of artifacts and creatures, even if they won't reduce Dargo's cost any further.

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.

The cost reduction for artifacts and creatures sacrificed earlier in the turn will apply even if you don't sacrifice any as an additional cost.