Travelers can normally purchase seats together while making online reservations. After selecting a flight and entering passenger details, the United Airlines website invites customers to view the seating plan and choose from the available seats.
If a person is unable to book seats together, it may mean that other passengers have already reserved the seats or the airline is holding back some seats for operational reasons. Customers can visit the Manage Reservations page of the United Airlines website closer to departure, to see if additional seats have become available. United representatives or flight attendants may also allow customers to change seats at the gate.