I am looking at a quad-plex that only has two water tanks and two furnaces, where I expected four (photo below). Seems like each furnace runs two units, same for the tanks. There is no way to fairly bill usage back to tenants as its not separately metered. I could include a flat fee in rent each month, but that seems like asking for pain and a good way to hemorrhage money. 

Has anyone found a sensible solution for this scenario?  How have you solved the problem? Ideas welcome.