In Commander, if your commander has partner, can both commanders contribute to the color identity of the deck?

In Commander, a deck's color identity is determined by all mana symbols and color indicators appearing on your commander(s). When you have two commanders via the Partner mechanic, both commanders are considered your commanders simultaneously, and the deck's color identity is the union of all colors found on either card (CR 903.4, 903.17d).

This means if one partner is a white/blue commander and the other is a black/green commander, your deck's color identity is white, blue, black, and green — you may include cards of any of those four colors (plus colorless cards) in your 99-card deck.

Partner is a keyword ability (CR 702.124) that explicitly allows a player to designate two legendary creature cards as their commander as long as both have Partner. The rules treat both as commanders for all purposes, including color identity calculation.

Example: You choose Tana, the Bloodsower (red/green) and Tymna the Weaver (white/black) as your commanders. Both have Partner. Your deck's color identity becomes red, green, white, and black — you can include any cards that only use those colors, but not blue cards.

Note: There is also a variant called 'Partner with [name]' (CR 702.124f), which only allows partnering with the specifically named card, but the color identity rule works the same way — both commanders contribute their colors to the deck's identity.