I have two separate tickets, one arriving SFO at about 5am, and one departing at about 9am. As the second is an international journey (I know, risky), I won't be able to do online check in. Can I check in airside at one of the United desks, or do I have to leave the terminal, and pass through security again? (or will the United agent at the start of the first ticket check me in for both?)