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If you have the ball position at (x,y) (after correcting the distortion), you can just write the projection equations:

x_px = f_x * X/Z + c_x
y_px = f_y * Y/Z + c_x

Where (X,Y,Z) are possible 3d coordinates in the world. As you have only two equations for three unknowns (X,Y,Z), this will give you the possible positions of the ball along a line. This line goes through the optical center of your camera and e.g. the point [(x_px-c_x)/f_x,(y_px-c_y)/f_y,1]. As you have the 6D transformation between both cameras, you can bring the two rays into one coordinate system and find a point with minimal distance to both of them. (don't try to intersect them, as they probably never have an intersection point)