The terms and conditions of international tickets are usually so that you need to stay out of the country for x # of days.
Usually, not showing up for a leg on your ticket will result in the cancellation of the rest of your ticket- so I wouldn't recommend building in the whole travel package into one carrier.
Also, if you are trying to nest a ticket inside another ticket, I'd recommend booking it on another carrier. For example.
Ticket one:
A-B march 2013, carrier one.
B-A aug 2013, carrier one.
Ticket two:
B-A march 2013, carrier two.
A-B July 2013, carrier two.
If you book both on the same carrier, you may face a cancellation if their revenue management department gets a hold of it for violating terms and conditions. Unlikely to occur but its a possibility.