As long as the two flights are on separate itineraries you can check in for both flights starting at 12:00 a.m. on the day of departure. If they are on the same record locater number, then I do not believe you can check in for the return flight until the scheduled departure time of the first flight. They may have changed this, but if I have same day returns, I always put it on separate itineraries so I can check in. If they are on the same record locater, just call and they can separate them for you.