They can be linked but that has no meaningful purpose. On separate bookings you have no connection and no protection, but in practice UA won't leave you hanging. Even if this were a single booking I'm not sure what your options would be, since if you miss the flight you've missed the flight. They won't hold it for you. Also, where the aircraft comes from really doesn't matter. You'll need to get off the plane and go through immigration and (presumably) pick up your luggage.