Technically the scenario described is "nested tickets". As long as all segments are used, this is legal and ethical. Back-to-back usually means the second portion of each ticket is tossed. Airlines consider back-to-back against their rules. I have done nested tickets with DL and and other airlines several times and never had problems. I have done it for personal and business travel using my corporate travel agent and also for segment runs. Besides the problem claiming mileage on the same account with NW and CO, you have to watch getting invol bumped to the other airline. This happened once to me with CO and AA nested tickets.

[This message has been edited by outoftown (edited 01-10-2002).]