Mixed cabin has to be booked by phone. Make sure they charge you the right number of miles too. I booked BKK to SIN (J) to LAX (F), and the phone agent initially tried to price it as BKK to SIN (J Saver of 30k miles), and then SIN to LAX (F Saver of 118k miles). I pointed out that the correct mileage should have been BKK > LAX (F Saver of 118k miles), the agent wasn't sure so they got a supervisor, and the supervisor booked it correctly as 118k right away.