Far easier to just have a single vhd per OS and add a boot entry to the primary vm that contains the required bcd. There is no (reasonable) limit to how many vhds you attach to a hyperV vm.
With multiple dynamically expanding vhds, you will minimise storage space.
Also, if separate, if one installation gets corrupted, it does not affect the other vms (other than a minor bcd boot tidy up needed.
Also, it gets confusing when you refer to vms when you mean vhds. VM is the virtual equivalent of pc, vhd is virtual equivalent of a drive that you attach to vm.
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