i think technically it is back to back ticketing. its definitely not illegal. i doubt its against coc because i think the recourse for the airline in back to back ticketing is to charge the full fare for both tickets. but if you already paid full fare, they really don't have any recourse.

as flyinbob said, i really don't think its the CSRs job to get angry at customers who break these types of rules. (the airline could simply take action after the flight is over as easy as they can before the flight started) in the way of taking away your ff miles and status, etc... but once again, since you paid full fare, there should be no recourse at all.

contract of carriage under ticketing:
C) USE OF COUPONS FROM TWO OR MORE TICKETS ISSUED AT ROUND TRIP FARES
FOR THE PURPOSE OF CIRCUMVENTING APPLICABLE TARIFF RULES (SUCH AS
ADVANCE PURCHASE/MINIMUM STAY REQUIREMENT) IS NOT PERMITTED. UA AGENTS
AND AUTHORIZED TRAVEL AGENTS ARE PROHIBITED FROM ISSUING TICKETS,
COMMONLY REFERRED TO AS “BACK TO BACK”, UNDER SUCH CIRCUMSTANCES WHEN
THERE IS OBVIOUS INTENT TO ABUSE AND/OR MISUSE RESTRICTED ROUND TRIP
FARES. AGENTS FOUND ISSUING SUCH TICKETS MAY BE LIABLE FOR THE
DIFFERENCE BETWEEN THE FARE PAID AND THE FARE FOR TRANSPORTATION USED.
UA HAS THE RIGHT TO DENY TRANSPORTATION TO PASSENGERS FOUND UTILIZING
TICKETS IN THIS MANNER UNLESS THE DIFFERENCE BETWEEN THE FARE PAID AND
THE FARE FOR TRANSPORTATION USED IS COLLECTED.

sorry about the caps lock, thats how it was on the site and i dont know how to make it lower case without re-typing it entirely