I've had a lot of same-day returns and always checked in online. The system will let you check in online for the outbound flight at the 24 hour mark. That's 24 hours before that flight. After you cross the 24 hour mark for the return flight, go back to OLCI and the system will ask if you wish to review the already checked-in segment, or the return segment that is now available for OLCI. Select the latter and you should have no problem.