See Conflict of Interest. Expanding the definition of "partner with" like banding on defense. You always get the tutor effect, but I'm tweaking the commander rule to be "You can have two commanders if they are both legal commanders, and either they both have "partner" or one of them has "partner with X" and the other is X."

I suggest a similar variant of the mechanic on Trenna, Elemental Summoner. Some considerations I made:

• To help differentiate "partner with <name>" from a more open version, I insert "any" as in "partner with any <characteristic>".
• I still require both to partner with each other so we don't get into "strictly better" issues where e. g. this is now a balance consideration for any future legendary Octopus, even without partner.

Can we make it so that partner automatically implies the "~ can be your commander." text, though, at least on legendaries? Well, I just want Oathbreaker rules for Commaner anyway, but losing that line on a planeswalker saves sooo much space - and requiring both seems like sooo much overkill.