Historical Rules · 12 July 2019
Rule 702.123f
Official source text from the 12 July 2019 Rules release
“Partner with [name]” is a variant of the partner ability. “Partner with [name]” represents two abilities. One is a static ability that modifies the rules for deck construction. Rather than a single legendary creature card, you may designate two legendary creature cards as your commander if each has a “partner with [name]” ability with the other’s name. You can’t designate two legendary cards as your commander if one has a “partner with [name]” ability and the other isn’t the named card. The other ability represented by “partner with [name]” is a triggered ability that means “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.”