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 one of them, not from both of them combined. Command Beacon's effect puts one into your hand from the command zone, not both.
Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren't included in the ability's reminder text.
The triggered ability of the "partner with" keyword still triggers in a Commander game. If your other commander has somehow ended up in your library, you can find it. You can also target another player, whether or not they have that card in their library.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Justyna Dura
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Spells that were cast before Alphinaud Leveilleur entered count. If Alphinaud Leveilleur was the first spell you cast this turn, the next spell you cast this turn is your second spell.
The second ability represented by the "partner with [name]" keyword modifies the rules for deck construction in the Commander variant and has no function outside of that variant. If a legendary creature card with "partner with [name]" is designated as your commander, the named legendary creature card can also be designated as your commander.
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 Alisaie Leveilleur and Alphinaud Leveilleur are your commanders, your deck may contain cards with white and/or blue in their color identity, but not black, red, or green.
To have two commanders, both must have the partner ability or corresponding "partner with" abilities as the game begins. A creature with a "partner with" ability can't partner with any creature other than its designated partner. Losing a partner ability during the game doesn't cause either to cease to be your commander.
"Partner with [name]" represents two abilities. The first is a triggered ability: "When this permanent enters, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle."
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.
An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.
Alphinaud Leveilleur's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.
