Aha, they don't allow mixing partners.
With the exception of 1) mixing AS with a partner, or 2) one way with one partner and the return with another partner.
That's what it seems to be after playing around on their website doing a few random searches (and that's all with a crying baby in my hand - do you sympathize?)
Now I gotta find out whether the AS phone reps are aware of these (unpublished, it seems) rules.
Who knows, maybe they're first cousins with US reps. I wish