After rumors suggesting as much, Twitter co-founder Jack Dorsey announced yesterday that he is indeed returning to Twitter in a day-to-day capacity. He will lead product as Executive Chairman of the company, while also keeping his role as CEO of the up and coming payment acceptance startup Square.

It’s an interesting move, and one that many have been questioning. Can he successfully do both at the same time? They both seem like full time jobs, but evidently Dorsey feels pretty good about it. Square investors? That might be a different story. Nicholas Carlson at Silicon Alley Insider reports: “Last night, one of Square’s top investors, Sequoia partner Roelof Botha tweeted, “To do two things at once is to do neither. – Publilius Syrus. Then Botha deleted the tweet.”

Note: The above tweet was in reference to another Sequoia-backed service – the much talked about Color, but it seemed appropriate to bring the point up again. For further context, know that Sequoia has provided funding for Apple, Google, Oracle, PayPal, YouTube, LinkedIn, Cisco, NVIDIA, Yahoo, Kayak, and Zappos, to name a few.

There are plenty of questions surrounding Dorsey’s return. For one, what does it mean for fellow Twitter co-founder Evan Williams, who has apparently spending less and less time actually dealing with Twitter, since he turned the CEO reins over to Dick Costolo last year. Erick Schonfeld at TechCrunch shares the following statement, obtained directly from Twitter:

Ev decided a couple months ago to be less involved day to day at Twitter. He continues to have a close relationship with the company providing strategic advice and, of course, he remains an active board member.

Meanwhile, Williams is reportedly considering a new startup, and as previously noted, has (along with other Twitter co-founder Biz Stone) yet to tweet about Dorsey’s return to the company as many other Twitterers have welcomed him back with open arms.

Even as Dorsey takes on a new/reprised role at Twitter, he hasn’t slowed down on the Square promotion so far: