I had the same the other day Thursday trying to book LHR - SYD having put in my husbands Exec no. and decided to reserve seats, it kept coming back to say check 2nd pax executive number, in the end i called the gold line, and the agent there suggested I take off my husbands Exec no. and then proceed with the booking which then worked, all the time the agent was on the line and being extremely helpful, paid and then when the booking reference came theough he was able to book all the seats for me. I then after all that was done went into the booking reference and added my husbands Exec no. to the booking, this has never happened before, obviously a blip which is annoying but hey ho Gold line came to the rescue