Unless things have changed over the last year, a careful review of the contract of carriage states the nested flights are not allowed. Still, I am sure that nested flights are not easily recognized by airlines and a lot of people have gotten away with it. For all of these there are other fliers who have lost their frequent flyers miles and status as a result of nested travel.

To avoid repercussions flyers have used two separate airlines, and not listed their frequent flyer numbers on reservations.