Multiple carrier routing query
 

I was looking for a fare MSN-ANU 10/21-10/30.
Travelocity, Expedia, and individual airline sites all gave similar $950ish fares.
Orbitz had a "multiple carrier" choice of $590ish using NW and AA.
Although AA.com does not show it, I was able to supply cs with the routing and it ticketed.
If I had not come across this @ Orbitz routing, is there a method enabling this option in the future?

Feel free to send me packing to THE PRIMER...