It required agent intervention because the system won't show you how many award seats are available on a particular flight, and we each had to use our own mileage accounts. So we worked in parallel, and you guessed it -- only one ticket went through.
The agent tried 3 different iterations, and I'm not sure how he did it, but he got us onto the same flights both ways.
(We're cashing in the miles we earned when we paid for first-class tix to Glasgow last year, so we had enough for first-class award tickets to Toronto!)
For all the horrible things you hear about airline award programs, I have to say this was a very good bit of customer service.