If I want to calculate the dipole moment of a dielectric cylinder of uniform polarization perpendicular to its axis, I could multiply the polarization by the volume of the cylinder, which is okay. But another method is to consider the cylinder to be a superposition of two cylinders of equal and opposite volume charge densities, with the two axes separated by a distance d, and then calculate the dipole moment by multiplying the d vector by the charge of the positive cylinder. But this is the same way we obtain the dipole moment of two point charges. I've been looking for some derivation or justification, but I couldn't find any. Could someone justify this method for me?