[cogman10]

Working under the theory that large dense things fall faster than small things, you'd expect that the large stone would drag the small stone. Much like if you attach a piece of wood to a rock with a rope and throw it in water, the rock will drag the wood down.

You'd probably argue that even though they are connected with a string they are still 2 things and there's a density gap between the two of them.

Now, if you try and figure out what makes a thing a thing, it starts breaking down. But if you've already have the working theory, then making an explanation isn't terribly hard.