Early stage, I think it's possible, though difficult. I did both for about 8 months before going full-time on the startup. A lot of early-stage tasks (market research, prototyping, learning the technologies you'll be using) can be done part-time before you quit your job.

Long term, I think you're really screwing your startup by trying to do it part time. It's not just a matter of speed. There's a qualitative difference between the work you can do when you devote all your attention to it, and the work you can fit around your day job. Most products need the former in order to solve a real need.