I've been told (more than once) that you cannot check in for the return until you have completed the outbound, for security reasons. I've tried this online a couple of times for turnaround MRs where I was within 30 (at that time) hours of both outbound and return.

It seems that others have had different experiences. Were these all return to the originating airport?

The one time I did get both was a SFO-JFK-SJC routing, and I'm sure it was because it appeared to be one continuous flight, and not a return flight.

Cheers.