Alisaie Leveilleur

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.

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.

The cost reduction applies only to generic mana in the total cost of the second spell you cast each turn.

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.

Spells that were cast before Alisaie Leveilleur entered count. If Alisaie Leveilleur was the first spell you cast this turn, the next spell you cast this turn is your second spell.

"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."