Armix, Filigree Thrasher

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 a creature is attacking a planeswalker, that planeswalker's controller is the defending player.

While resolving Armix's ability, you can't discard more than one card to give more than one creature -X/-X.

If the discarded card is an artifact card, it will be counted as you determine the value of X, provided it's still in the graveyard or, optimistically, on the battlefield under your control at that time.

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.