A good travel agent would never do a back to back on the same airline. Airlines look for the following 6 things to see if back to back might be going on:
1)Same passenger name.
2)Same ARC number from issuing orgin.
3)Same FF number of passenger.
4)Same form of payment(ie same credit card).
5)same or similar dates and itin involved.
6)same date of ticket issue.

If 3 or more of the above click, the airlines will check into this.

If you are unable to fly on two different airlines then one should book one ticket with the travel agent and have the agent or yourself book the other ticket directly with the airline, or on the airlines web site or with a different travel agent.

If you do not have access to a travel agent and need to do a back to back try to do the following so the airlines don't try to confiscate your ticket at the gate(they have done this in the past).

1)Buy you tickets from different people/companies.
2)Change your first name slightly or use an inital.
3)Buy your tickets on different dates.
4)Use two different credit cards.
Change the return dates.

If you configure the tickets so you end up using all 4 segments, it is not a back to back. Some of the airlines might disagree but if you read the rules it is legal.