MP Program Rules now prohibit the old practice of "switch at the gate", to accrue mileage on one carrier while using the status benefits associated with another account. Certainly was possible in the past, many used to do it, but now explicitly forbidden. Rule 16(b):
Members receiving benefits at time of booking, check-in, travel or other activity in which Program benefits are provided, including but not limited to premium seating, fee waivers, additional baggage allowance, lounge access and other travel benefits must maintain their MileagePlus account number throughout the entire reservation and any subsequent reservations created from the original reservation, and for the entire duration of the original and future travel associated with these reservations. Further, if benefits are obtained by a companion traveler by virtue of a Member's status or other benefits and such Member is no longer traveling on their original reservation, any companion traveling on the original reservation will no longer be eligible for the Premier and other benefits obtained as a result of the Member's status or other benefit allowance.