Halana, Kessig Ranger

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.

Halana, Kessig Ranger's triggered ability goes on the stack without a target. While that ability is resolving, you may pay {2}. If you do, a second ability triggers and you pick a target that will be dealt damage. This is different from abilities that say "If you do . . ." in that players may cast spells and activate abilities after mana is paid but before damage is dealt.

For the creature that entered the battlefield, use its power as Halana's reflexive triggered ability resolves to determine how much damage is dealt. If it's no longer on the battlefield at that time, use its power as it last existed on the battlefield.

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.

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.