Haldan, Avid Arcanist

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.

Haldan's ability applies only while it's on the battlefield. You'll be able to play cards you exiled with fetch counters before Haldan entered the battlefield, and if Haldan leaves, you won't be able to play cards that were exiled with fetch counters until Haldan returns.

A creature card that you exiled with a fetch counter on it may be cast as a noncreature spell if a rule or effect (such as that of adventurer cards) allows you to do so.

"Partner with [name]" represents two abilities. The first is a triggered ability: "When this permanent enters the battlefield, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle their library."

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 Haldan and Pako are your commanders, your deck may contain cards with blue, red, and/or green in their color identity, but not cards with white or black.

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 who might have that card in their library.