co.com is biased towards selling its own seats. While you used to be able to book what you describe, you can no longer do this in an automated way. You can call them and get it, or if you want it online:

1. Hold an itinerary with your outbound, and any return.

2. Call the Electronic Support Desk (800-300-1547) and ask them to pop in the return flights you want.

It should fare out the same as Expedia.