Elon Musk has simple advice for Twitter's new CEO about running two companies: Don't do it

Jack Dorsey joined the elite (and very small) club of CEOs who run two companies this week, now that he is the boss at both Twitter and digital payments start-up Square.

But the advice from one of the other prominent members of the dual-CEO club is not very inspiring: don't do it.

During an appearance at Vanity Fair's New Establishment Summit on Tuesday, Elon Musk was asked about his thoughts on doing double-duty at two companies in light of the Jack Dorsey news.

"I wouldn't recommend running two companies - it really deceases your freedom quite a lot," Musk replied.

Musk knows a thing about running two companies. The South African-born entrepreneur is the CEO of electric car maker Tesla Motors and the CEO of space exploration company SpaceX.

Musk said that he spends about 70% of his day focused on engineering and design.

He estimated that he spends roughly 2% to 3% of his time doing press stuff, such as his on-stage appearance at the Vanity Fair summit. And apparently even that is too much. "I'm going to reduce it even further to only about 1% of my time," said Musk.