I don’t think that was probably meant as a hard and fast rule, just a reference to an example of the type of constraints they have to deal with.
It doesn’t seem at all unlikely that there would be instances where there would be conflict preventing overlap, and instances where there wouldn’t be.
precisely. Crusher appearing on prodigy (as a hologram!) doesn’t really seem to me like an issue for Picard. On the other hand, I can’t imagine crusher appearing on LDS without their team checking on where the character should be at that time.