Merry, Warden of Isengard

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 Frodo, Adventurous Hobbit and Sam, Loyal Attendant are your commanders, your deck may contain cards with white, black, and/or green in their color identity, but not blue or red.

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.

"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."