Originally Posted by PVDProf

You can usually go to the partner website and do a dummy revenue booking to see the seat map for the flight. To actually get those seats, you would book the ticket through united, who will give you a record locator for the other airline (not the same as the UA locator). Then you can call the other airline with that locator and get assigned a seat. (Note, however, that not all airlines let you assign seats ahead of time, and many that do block bulkheads until day of departure as they are often held for families with post-birth babies in need of bassinets.)