You state that the panels are right next to each other but you do not mention how far they are from the meters. In fact, you never even mention using any meters at all!
If the panels are right inside the home close to the meter location (w/in 8' in WI) then there is a cheaper/easier method than already mentioned. Use a double meter socket with 4/0 Al or 2/0 Cu up a single mast. There is no need to have OCP within the meter as long as the service entrance conduit is short. Run two 1 1/4 PVC’s from double meter to each panel inside. Use 3 #4 Cu or #2 Al conductors from each meter to its respective panel. Do your neutral to ground bonding in each panel (usually a green screw provided with the panel). You only need #8 Cu to run from the water pipe to each panel. One wire will suffice but you have to use an irreversible splice (such as a crimp) when you turn that one wire into two to attach to both panels. The wire to the ground rods (you have to use two in WI) also only needs to be #8 but, if the wire is not protected outside, you will need to use at least a size 4 Cu. Bare #4 copper works best for most residential ground rod wires because it doubles as a readily accessible grounding location on the outside of the home for such things as telephone and satellite dishes.
After rereading your posts, I don't think I completely understand your situation. Since you say that you are an electrical contractor, there is absolutely no way you could have forgotten about installing a meter socket. That would be about as likely as an electrician forgetting how to install a lightbulb! Apparrently, there must be a single meter elsewhere on the property and the rental units are not separately metered. But if that were the case, now would be the time to rectify the situation which, of course, you should also know. Something is not adding up. Please fill us in on the secret.
