Steve Jobs managed to be pretty effective as CEO of both Pixar and Apple at the same time for a while. Admittedly, his involvement at Pixar was mainly strategic whilst he was running Apple, but still.
Though Jobs was obviously the exception to so many rules.
I also don't buy the example of Musk and others, I think we have context as to how much better each of these companies would have run if they had put 110% of their efforts into just one.