The short answer to your question is, no. You would only have one pistol at a time.
However, if you consider swapping stocks and barrels back and forth, both receivers would need to be registered as pistols, unless you want to keep writing letters back and forth to the MSP and submitting paperwork to the local police.
Register them both. You'll have the option then, and be covered, as to which one you want to have in the pistol configuration at any given time.
If your seeking additional information about the lawful possession, transportation, and carry of a registered "Michigan pistol", you can pose you questions on the Michigan Gun Owners forum in the Open Carry section. You'll get the answers your seeking there.