IME, you need certain attitudes towards life to get good at both.
You need to be a visionary AND an engineer. Usually people are one or the other and prefer it because they suck less at it. Hence, they eventually get great at it. But this means the other suffers. To be in the designer+engineer category, you first need to figure out what you are good at (essentially what you spend a large chunk of your time on - dreaming or coding) and then what you are weak at.
Then practice doing the weak thing for a couple of years.
Pretty soon, you are a designeer.
Problem is, in the startup world, I still can't figure out WHY you should be both when you can hire people to complement your weakness. Steve Jobs was obviously only a visionary and he, through practice, became great at it. Dennis Ritchie, an engineer.
Unless, of course, startup is not your endgame. Unless your goal is self-improvement powered by a zesty thirst for knowledge. In my limited knowledge, though Learnado Da Vinci fell into the designeer category, he was still very much an idea person (visionary) than an engineer - mainly because he procrastinated like crazy with his projects (for years, at times). This shows that he preferred conceptualizing the project and loved cultivating the vision rather than actually implement it.
Thoughts? Please give me some feedback, this stuff is important.