The gravitational force between two masses (say the Earth M_{E}, and a ball M_{B}) is given by Newton's Law of Universal Gravitation, F = GM_{E}M_{B}/r^{2}, where G is the Universal Gravitational Constant, and r is the distance between the center-of-mass of the two objects. So, if ball 2 has twice the mass of ball 1, then the gravitational force on ball 2 is twice that on ball 1.

The acceleration depends upon both the force and the mass, F = ma. Since F_{2} is twice F_{1}, and M_{2} is twice M_{1}, the acceleration on the two different masses is the same.