Nested tickets
are allowed.
Back to Back tickets
are not. However, splitting it up between 2 airlines just means that it's unlikely you'll get caught; it doesn't make it ok with the airlines. Delta would rather get the high price fares from people who can't/won't do back to back ticketing and the partial revenue from those who do than get less revenue by allowing back to backs.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by ATLJV:
Assuming that you fly all of the segments, the airline actually loses here. due to their on rule. Assume the fare RT fare is $300, then DL is sharing getting $300 and NW $300. DL could have $600 if the nested tickets were allowed.</font>