Wolf:
The issue would be that if they manifested an intention of kiddushin, we would probably say that the actual intention is irrelevant since “devarim shebileiv einum devarim”.
Now, I suppose you might argue that here everyone knows that this manifestation of kiddushin is not real- and that makes it not a manifestation. Maybe it is like if they are actors in a play. I wonder what the halacha in such a case.