I'd just tell Exhibitor A to go ask Exhibitor B if they really wanted to know if ExB was going to the next show. That way Ex A can lie (or not) to Ex B and you're not in the middle of it. They can work out their own dynamics.
LOL! That is a good one! It sounds like a genetics chart! You are very right, if they want to lie/hide info to each other, that is up to them, but I'll keep myself out.