An ability that triggers when a player spends mana to cast a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
Gilanra, Caller of Wirewood
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.
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Jehan Choo
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