The equation to find combined centre of Gravity, is explained very well here:

https://www.youtube.com/watch?v=KMDY-hexpZ8

mass1 = vol1 * density1 , centered at point mass m1.
mass2 = vol2 * density2 , centered at point mass m2.
d = distance between (center of mass of m1) and (center of mass of m2).
x = distance from (center of mass of m1) to the point on the line between m1 and m2, which is the combined centre of Gravity.
m1 * x = m2(d-x)


x = (m2 * d ) / (m1 + m2)

- Brian

They say is the "same" for torque. (No inverse square law needed.)
(x is not necessarily the x-axis), so x,y,z coordinates of the points may be used, or a cplane making d along the x axis...)