I do that. I'm a freelance developer part time, and a freelance musician part time. It's difficult, and way more work than just doing one or the other. You have to constantly guard your ability to do it -- if you let them, people will demand your absolute full time devotion to them. But I've found when you lay out th rules early on, they usually accept that they can't have you body and soul. If not, they weren't a good employer for you anyway.
I suspect that it requires other ancillary skills like negotiation, self-marketing and sheer chutzpah which I don't necessarily have all at the same time.
I like programming, but sometimes I wish I could do it only a few days a week and spend the rest doing something where I wasn't hunched over in front of a box 13 hours a day. Sales! Photography! Helping injured animals! I've never done anything that didn't involve sitting alone a desk all day...