I had another boss at the same company that filled a NeXT cube to capacity with motherboards. For the sake of a good story I will say he had them all running under the same Mach kernel. He described the simple thing he did to get it running, but I have forgotten.
The additional boards were netbooted via bootp/tftp, so the additional boards could run the same kernel as the one the hard disk (and/or MO drive) was connected to. Of course, this is a distributed memory machine - not much different than running three diskless Cubes booting from a fourth machine.
Technology was moving really fast in those days.