In such a situation, I would expect there are high chances of being subject to the SSSS process at or on the way to the gate for the US-bound flight on UA at the international gateway airport for the US-bound flight. In such situations, the security contractors working the UA flights generally have not just skipped SSSScreening me. I've also managed to avoid some such situations in one way or another. One way is by asking for UA to uncheck me in for the US-bound flight and then try to re-check me for the flight -- but it's far from being a reliable way to avoid such SSSScreening.