I've been working for startups a long time (over ten years) and I have only met one person that I would consider a great designer and developer. I don't think it is because others aren't capable but rather aren't interested or forced to. This type of person is definitely a rare breed which is good. I think a lot of people say they would like to fall into this position but in reality probably not so much.

Please keep in mind that I have met quite a few people that dabble in everything but a true professional at both is quite rare.