Marriott Bonvoy, then Marriott Rewards, and United Airlines MileagePlus launched a program called RewardsPlus back in 2014 that was modeled after what Starwood Preferred Guest (SPG) and Delta Air Lines had in place for reciprocal elite benefits.
Marriott Bonvoy elite members can get a status with United MileagePlus and vice versa. There are also be some additional benefits when converting points to miles both ways.
Marriott Bonvoy Titanium & Ambassador United Airlines benefits:
Marriott Bonvoy Titanium and Ambassador members are eligible for United Airlines MileagePlus Silver benefits that are rather limited.
Silver members are eligible for one free checked bag and Economy Plus-seating if available at the time of checking in. The complimentary upgrades to domestic first are highly theoretical and can happen once in a full moon to/from leisure destinations.
United Airlines MileagePlus Gold, Platinum & Global Services Marriott benefits:
United Airlines MileagePlus Gold and higher members are now (downgraded) eligible for Marriott Bonvoy Gold that comes with equally limited benefits as MileagePlus Silver.
Marriott Bonvoy Gold members are eligible for “enhanced” room upgrade and some bonus points. No lounge access or complimentary breakfast are included. There is also space available 2PM late check out.
Hyatt’s partnership with American Airlines and Accor’s partnership with Air France-KLM allow members to earn bonus miles and hotel points through their partnerships. Marriott’s partnership with United, however, is not that extensive.
It does make sense for Marriott and United elite members to match their status for the limited reciprocal benefits. You would image that airline and hotel elite members already have their preferred partners.