You would have to implement single system image abstraction, if you wanted more than a networked cluster of M1s in a box, in the OS using just software plus virtual memory. You'd use the PCIe as the interconnect. Similar has been done by other vendors for server systems, but it has tradeoffs that would probably not make sense to Apple now.

A more realistic question would be what good hw multisocket SMP support would look like in M1 Max or later chips, as that would be a more logical thing to build if Apple wanted this.