For Elon Musk, working at only two companies would be a substantial reduction in employers. He currently works for Tesla, SpaceX, The Boring Company, Neuralink and OpenAI.
* Update: Apparently Elon quit OpenAI. So let's not count that one. But even 4 employers is still a lot!
Elons probably not an employee, directors are different
Directors really aren't different and in fact one would argue have access to even more sensitive information. In that light, work 2 jobs if you want! I guess the only issue is that a director would have informed his official employer of the engagement.