It's one thing to be CEO of two companies that are doing well, with a well-established strategy and strong teams of A-grade players that are executing well.
What I don't understand is how Jack Dorsey could believe he could successfully manage two companies, Square and Twitter, at the same time at such critical points in their growth. I just shake my head at the unbridled hubris and over-estimation of one's own abilities that one must have to make this decision. It's very frustrating, because I believe both companies could be so much greater at what they do; they've just touched the tip of the iceberg.
It's hard enough to turn around one company successfully. But even if Dorsey is successful in the end, splitting 50% of your time somewhere else is a needless way to slim down your odds, kind of like smoking a pack of Marlboros a day and hoping to make it to 100. It happens, but that doesn't make it a good idea.
Or to paraphrase from elsewhere: "It could be that the purpose of Dorsey's career is to serve as a warning for others."