I doubt that'd work. My client is set up as multiple merchants (different companies from a legal pov) but with the same payment provider, going over the same pipes, etc. Even so, the way the payment servers were set up, they could only mix up the merchant config if the "wrong" one also happened to be on the same server.

They have dedicated servers, so I'd expect McDonald's and other big chains to have them, too, which means that you couldn't send payments no anyone outside the mother company.