There's no thermodynamic reason that it couldn't work the way you want it to, but the physical connections and electronics controls need to be designed to accom

There's no thermodynamic reason that it couldn't work the way you want it to, but the physical connections and electronics controls need to be designed to accommodate that, either with direct connections on the outdoor unit or via a 'branch box' that serves as a connection point for both the refrigerant and electronic control circuits. It's most likely that the installer was correct in that the outdoor unit you had maxed at two and did not have a smooth expansion path.