Edit: three companies, but I guess he isn't in Socialcam's core team (or is he?)
I spent most of Monday at Justin.tv / TwitchTV. At TwitchTV I mostly work on strategy and management issues. My cofounder Emmett Shear runs the company as CEO along with a great management team including our COO Kevin Lin, who runs all the partnerships and operations, and CRO Jonathan Simpson-Bint, who is the founder of IGN, and runs all biz dev. Operationally the team is excellent, the company is growing well (almost 60 people now between two offices) and my services aren't really required for operational issues. Monday night I work on Exec stuff as well.
Tuesday is YC day. Since we're going through YC for Exec, I'm down in Mountain View anyways. I hold office hours during the afternoon most weeks, and then attend the dinner with everyone else.
Wednesday -> Sunday I'm working on Exec full time, but occasionally do calls for JTV/TTV. I also stop by the JTV office a few times a week just to hang.
Socialcam is being run by our other cofounder on JTV, Michael Seibel, along with two engineers formerly from JTV. It is fully spun off as a corporate entity now. We still have a great relationship and they are part of the JTV family, but I don't have an active role. They are off to the races though and on the way to be extremely successful.
UPDATE:
Re-read this and it makes it sound like I work an insane amount. I don't want to make it seem like I'm Jack Dorsey-ing it. I take time on weekends and nights to relax and have a social life. I think working at a sustainable pace is important. Luckily (for me), I enjoy working at so many things because they give me a chance to exercise / learn different skills (strategy and growth at JTV, programming and pitching at Exec, talking about business ideas at YC).
Even Jobs had to neglect huge portions of Apple itself to focus on developing new products like the iPhone.
One reason it probably can't really work very well: http://www.paulgraham.com/top.html
I can think of multiple counter-examples as well. People who are trying to run multiple organizations and doing rather poorly (as judged by results). I wouldn't want to single them out publicly though. Besides, it's rather difficult to judge the cause and effect for things like this.
I'd love to have this question answered.
Edit: Apparently, he just changed his role at Justin.TV to an advisor. So I think most of his time is spent on Exec, while on the side advising YC companies, Justin.TV/Twitch.TV, and Socialcam. But still definitely a lot for one person to manage.