Is it possible to become 'excellent' at both design and coding?

Today I left my position at a tech startup (to join a different tech startup). My former boss and I have some differences of opinion, but I respect him a LOT, and he gave me some parting advice that really stuck with me.

He said, "You're going to be successful. You're very good at two things [design and front-end development]. You need to focus on ONE of them, and get from 'very good' to 'excellent'. Then build your career on that."

My career thus far (5-6 years) has been based on straddling the fence between the Photoshoppers and the code wranglers, and I'd like to think that I've gotten to where I am because of that mixed skill set. But my boss wasn't the first person to tell me this. I've had people from both camps suggest that I pick one discipline or the other and really become an expert.

A couple questions:

1) Am I selling myself short by trying to be good at multiple things, when I could be an expert at one? 2) Is it even possible to become truly excellent at more than one thing in one's career?

And a bonus question:

*) Are there any examples of 'generalists' who have built and/or led successful startups?

Why not become an expert at both? If you ever decide to have your own startup, being able to do the work yourself vs having to pay something else to do becomes your greatest asset in the initial stages.

Only one way to find out - lets see your work.

More seriously though, I can relate. I'm in a pretty similar position as your own. The biggest advantage you have is that you can bring ideas to life, 100%, without ANYBODY else. And that is very,very powerful. You are not limited by your skills or abilities, but only by your ideas and personal drive.

I think there is simply too much stuff to know about in both coding and design. There is simply not enough not time to become really knowledgable in both.

However, I am not sure if you really need to be super knowledgeable to benefit from knowing both sides of the fence. I’m sure it’s possible to become really good at a subset of coding and design.

By the way: these are just my musings. I’m very early in my career myself. Personally I’m aiming to become effective at programming relating to the user interface of things, and design wise I focus my efforts on interaction design, information architecture and visual design. I want to learn more than what I just mentioned, but this I what I aim to be my core.

Unfortunately, there will always be a multi-classing penalty of sort. (I'm one of those people who code and design for web development, but masters neither.) For design, I've learned that the essential trait is to set one's mindset and philosophy on problem solving (as well as have an intrinsic taste for art). For code, I believe it is quite similar. Except that standards and practices constantly change. As a designer, it is your goal to meet and deliver these standards. Personally, I find it hard to be extremely good at both although I still consider myself competent.

My personal answer your question:

1. Some companies/start-ups value designers who have penchants for design while still being technically competent. e.g. someone who is familiar with grids, typography, art movements. While at the same time is familiar to version control, knows the basic structure of an app and the ability to build it single-handedly. Although I think that they serve a better role as project managers.

2. I have yet to witness one. If ever I meet one, I'd like to know how they were able to balance their daily information consumption.

edit: spacing