Well one ball is going to be heavier than the other ball.
How do you tell when they're in the same sack?
One last weigh with them separated from each other. Which is not available.
I'm sure this is a sorting problem where logic has pointed out that only certain balls were ever on the heavier side.
In the midst of moving 3 around, keeping track, etc. logic has determined the single ball.
Not a separate weighing.