With the light ball I was free to be more dynamic and less limiting with the movement, so I found that easier than the heavy ball. The main thing I considered would be that, although it would have to hit the ground on the same frame as the light one, it would have to have much fewer bounces following it. However I found it a bit difficult trying to determine exactly how many, and how high to make them. Sadly I don't know much at all about physics so in the end it was all just simply my judgement of what looked more realistic.
I'd have liked to have added a resulting "roll away" effect so that the balls took up more of the three-dimensional space, and I also am considering adding in some squash and stretch. But I shall leave it as it is for now.