I would imagine that the "host" would be diskless and would treat the "guest"'s boot disk as its boot disk. It's effectively the same as swapping the guest disk into the host, but all its other peripherals (ports, sensors, etc.) come along as well.

This is why I was asking whether there was ever a dock made for a specific computer that had this feature: it requires the guest's disk's OS to have drivers to let it boot the host—or even hardware compatibility such that docking the "guest" would cause it to hibernate to disk, and then boot from that same hibernation image on the host. That basically requires the host and guest [the dock and the portable] to be made together.

(And that's not even getting into weird things you could do with live process migration and NUMA and memory hotplug if you want to make both the host's and guest's processors available without an effective reboot/hibernation/sleep step.)

An aside: before it was revealed what exactly the Nintendo Switch was (a portable with a variable clock rate that overclocks itself when plugged into a heatsink dock), I had a suspicion that it might use this architecture: a portable that just turns itself into a disk and Bluetooth controller for a more powerful non-portable when plugged into it.