Chapter 2 - Kinematics: Description of Motion - Learning Path Questions and Exercises - Conceptual Questions - Page 59: 19

Please see the work below.

As we know that both the stones travel with constant acceleration that is $9.8\frac{m}{s^2}$ in vertical downward direction. On the other hand, their relative acceleration is zero i.e they move with constant velocity with respect to each other and hence the distance between them will also be constant.