I think the answer is very much "yes" based on SWISS website and the above reported experiences, but wanted a pulse check on the following guest / companion scenario:

PAX 1 flying IAD-ZRH-MAD in LX F / LX J
PAX 2 flying IAD-ZRH-MAD in UA J / LX J (on a separate ticket from PAX 1 of course, but on the same ZRH-MAD flight)

PAX 1 and 2 are on the same ZRH-MAD flight. I believe PAX 2 would have access to ZRH FCL based on PAX 1's inbound F flight, and the fact they are on the same ZRH-MAD. Correct?