Effect of Coefficient of restitution of two balls

Suppose I have a tennis ball of COR 0.5. This means that when it bounces on an immovable object (like a floor), then it will return with 0.5x the velocity, right?

Now suppose I also have a football of COR 0.8. If I bounce the tennis ball on the football, how much energy will be lost? Say the football is on the ground and I drop the tennis ball from a height of 1m, how high will it bounce (assuming that it bounces straight up)?

Assuming that the COR remains constant for the velocity range, it's a simple multiplication for subsequent bounces: $$v_{i+1} = \epsilon\cdot v_i$$ or resolved for the COR $$\epsilon = \frac{v_{i+1}}{v_i}$$. Remember that $$E_{kin} \propto v^2$$ and you see that the ratio of kinetic energy is the square of the COR (mass stays constant so cancels out)