We have seen oil tanks installed with piping connected in series (single fill and vent pipes shared by two tanks) that have led to leaks from the tank top or leaks from tank welds that have burst during fuel delivery. CommTank recommends installing oil tanks with separate fill pipes. This will ensure that the tanks are never over pressurized during filling. Connecting vent pipes in series is acceptable, as the tanks are filled separately, and will never develop back pressure.