A commander with the Partner ability can only be paired with another commander that also has Partner. This is established by CR 702.124b, which states that two legendary creatures may both be designated as commanders if both have partner. You cannot simply pair any commander with a Partner commander just because one of them has the ability.
There is also a variant called 'Partner with [name]' (CR 702.124f), which is even more restrictive — those commanders can only partner with the one specific named partner, not just any other creature with Partner or 'Partner with.'
Additionally, some cards use the 'Friends forever' keyword (CR 702.124k), which works like Partner but only allows pairing with another commander that also has Friends forever, not with regular Partner commanders.
Concrete example: Tymna the Weaver (Partner) and Thrasios, Triton Hero (Partner) can both serve as your commanders because each has Partner. However, you could NOT use Tymna alongside Bruse Tarl, Boorish Herder who has 'Partner with Akiri' — they must partner with their specifically named counterpart.
The bottom line: Partner only unlocks two-commander play when both commanders have a compatible Partner-type ability (Partner + Partner, 'Partner with' + matching name, or Friends forever + Friends forever).
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.