Elon Musk is perhaps the most famous living dual-CEO in the world, running both electric car maker Tesla and space transportation outfit SpaceX at the same time. He was joined yesterday in the chief executive multitasking club by Jack Dorsey, who will now lead both Twitter and mobile payments company Square simultaneously. But don't ask Musk how or why he does it. In fact, don't take any advice from him on running two huge multi-billion-dollar corporations. He says it's a bad idea.

"I wouldn't recommend running two companies," Musk said today onstage at the Vanity Fair New Establishment Summit during a panel with Y Combinator president and investor Sam Altman and New York Times journalist Andrew Ross Sorkin. "It decreases your freedom quite a lot." Sorkin wanted to know what it was like running two companies, especially now that Dorsey has two CEO positions to handle.

"It decreases your freedom quite a lot."

The 44-year-old serial entrepreneur and prospective Mars nuclear terraform specialist is known for his inhuman work ethic and the demands he puts on Tesla and SpaceX employees. Musk flies many times a week between the Hawthorne, California SpaceX headquarters outside Los Angeles and the Tesla factory in Fremont near San Francisco. He also has five children, and apparently somehow has the time to take them camping.

Musk elaborated on his work day at the New Establishment panel, saying it probably doesn't resemble what most people think. Musk says he spends about 70 percent of his time on engineering and design, and about 2 to 3 percent on press events and speaking engagements. He's probably going to cut that down to about 1 percent, he added. In other words, prepare to segment your day into individual percentage points, Dorsey.