Ardenn, Intrepid Archaeologist

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 generic mana 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.

Other players and permanents controlled by other players can be targeted by Ardenn's ability. This can cause Equipment under your control to be attached to a creature an opponent controls. Auras and Equipment that say "you" still refer to you, as you still control them. Ardenn's ability can't cause an Aura or Equipment to be attached to something it couldn't be attached to.