He however owns a significant percentage of both companies.
Countries already do this to pressure countries and/or individual entities. Trade embargoes are against entire countries, trade sanctions are over certain industries/companies/specific individuals.
I clear international freight through customs for a living, when I have a shipment containing something coming from/made in a sanctioned country I have to search the OFAC sanctions list for every party mentioned on the shipment https://sanctionssearch.ofac.treas.gov/
China, or Fictionaltopia, could easily put pressure on one company to influence another if they have a significant investor in common.
I imagine if Mr. Musk wants to keep his shiny new Tesla factory in China, he'll make sure this won't be used to circumvent the Great Firewall.