I have a square that motion tweens and rotates slightly as it comes to a hand. Next, I need to twirl the hand upward with the square in it, but then the hand must release the square. Now, I could join these two graphics into one movie, but is that what I want to do? Or is it best to rotate them separately?
You can easily do either.
Personally, I have found it simpler to join objects into one movie and tween that for the period they need to move together, and then break it into separate layers again when they need to move separately. The trick is to be sure that the objects don't jump position at all when you switch.