Wish I'd read this thread before two overly long chat session with UA and a Twitter exchange and phone call with TK. Same scenario: two TK flights on a multiple-carrier reservation booked with UA miles. UA could only provide 016 ticket numbers but TK website requires 235 numbers to make seat reservations, and TK phone agent told me I'd have to wait until check-in for seat assignments (for BC). Was about to call again but then saw this thread, downloaded the TK app, and made the seat reservations just like that. Thanks, fbcooper and milepig!