Single conductor used for grounding two wire receptacles
Correct if I am wrong, but didn't the old code books allow for such a
thing. That you could connect to the nearest water pipe, (not plastic).
Then it change to the grounding conductor had to be run all the way
back to the service panel.
I can't point to anyhing in the code that saids you can't do this.
But I would make up my splices in a U.L listed junction box. Reason,
all 120/240 volt circuit are splice together inside j-boxes/oulet boxes/
If you are worry about the single conductor be damage, you could
could install it ENT tubing. But I wouldn't, I would label it, so others
would know what this wire is used for.
Now will hear from the rest of the guys.
Please note, the above is only my opinion.