The gravitational force between two masses (say the Earth ME, and a ball MB) is given by Newton's Law of Universal Gravitation, F = GMEMB/r2, 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 F2 is twice F1, and M2 is twice M1, the acceleration on the two different masses is the same.