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

This example sums it well: Steve Jobs.

Whichever one you pick, keep in mind that your long term goal is to go from micro to macro.

Yes and no. You can't be amazing at programming from the kernel up to the front-end and be amazing at designing from the front end all the way to marketing and UX. Reminds me of Forrst. About six of us had checked just "developer"; seemed like almost the entire population had checked both "designer" and "developer" and were proceeding to ask really basic programming questions.

On the other hand, there is seemingly limitless employment for people who can both design and implement a website. But that employment doesn't pay as well as just one or the other, even though it's easier to do just one or the other. The people who need these skills wrapped up in one person need it that way because they're too cheap to hire two people. The people who can afford someone special can afford two or three someone specials. In fact, the people who need you to be both probably would like you to write the backend as well, which you've sort of implied you don't do. So if you're in the market for employment with two decent skills, you stand good odds of being outcompeted by people with one excellent skill.

The problem with this line of reasoning is, where do you draw the line? I think the distinction between front-end developer and designer is pretty tenuous. If I show a developer a binary tree and ask them to calculate the height mathematically, I expect to hear the answer "log base two" without delay. Most of the front-end developers I've met have encyclopedic knowledge of Javascript, the DOM, several JS libraries and the various browser quirks, but could not solve a pure coding challenge offline with no time limit and their choice of printed technical literature. If you fit this front-end developer profile, but can also design, you belong to an interesting and fairly small class of people who really can do essentially the entire front-end themselves. I laud these people and I'm not at all convinced that to be effective at one you have to give up on the other. On the other hand, if you're a kernel hacker on the weekend and doing front-end development and design during the week, I think the advice is probably sound. You're not going to be able to be one of these front-end wizards while also doing totally unrelated stuff.

I consider myself a generalist, and have really enjoyed working across several domains (primarily design and programming) - it is quite possible to pursue a profitable career while not fitting entirely into the boxes that bosses and hr would like you to fit into. We've just launched a startup over at www.coolplaces.co.uk for which I did the design and programming, but I am not a reliable judge on whether it is excellent or not :) FYI I started in fine art, and have worked for a living in design and programming.

There are many advantages to being familiar with both programming and design, and many similarities between the disciplines at a deep level (thinking through a problem holistically, balancing constraints, incorporating and developing ideas from clients). I think those advantages more than pay for any disadvantages which come with not being a specialist in one subject. In fact knowing something about several disciplines will often help inform your work in both and allow you to bring ideas form one to another - that's why many education systems require students to pursue a broad coarse of studies alongside their more narrowly focussed specialism even in higher education.

As an example of cross-fertilisation helped by working on both, for design I use version control to store my work, which is a practice imported from programming which I think many designers would love to have. For most designers, the equivalent is to keep 50 files in a folder marked design-1, design-2, etc. There are also collaborative systems like ticketing systems broadly used in programming which apply very well to managing client expectations and letting clients track your work and prioritise certain bits of it.

Looking at present day startups, I think you could say that a startup like 37signals qualifies as having many generalists - whether you classify them as excellent or not is up to you, but they are certainly successful. For more examples, look at the banner on this website http://designerfund.com/ - I'm sure many of those companies contain many excellent generalists that you could take inspiration from.

Looking back to the past for examples, there are many many examples of polymaths who have excelled in several disciplines, like Leonardo da Vinci, who was both an excellent engineer (often overlooked), inventor, and hugely creative and influential in the art world (which he is of course famous for). Which I'd say is definitive proof that your boss's conclusion is wrong in general. It is still possible your boss is right about you and your future direction, it seems unlikely if you are passionate about both areas that you can't grow in both, however it is possible that you are far more suited to one than the other and would benefit from specialising. Without seeing your work, I don't think anyone here could say.

Personally, I would simply take note of what your ex-boss has said, and continue to do whatever interests you, and work in areas which you feel competent in, and enjoy. If you find that throughout your career, lots of people are advising you to choose one path over another, it should lead you to either try to improve your skills in the area they highlighted as less than stellar, or move toward specialisation if you don't feel any desire to improve in that area. If your clients continue to be happy with your work in both fields, why would you feel the need to specialise?

I don't think you can extrapolate from this particular bit of advice to you a general statement about the possibility of impossibility of generalism, and I think given the evidence from the past, the present, and other posts on this board, the answer to your central question above:

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

Is a definite YES. Whether you will or not is up to you.

That seems like a very simplistic question, and adrian and seoxy are trying to explain why that is so. There are MANY facets within both coding and design. It would be simplistic to assume you could be excellent in all facets of either one (because they are so broad). So why can't it be possible to be excellent at certain aspects of both coding and design?

I don't think it's possible to become excellent at both being a designer and a developer but I do think it's possible to become an excellent designer/developer and the distinction is important.

First of all, I think there's been a devaluation in the term "excellent"; every startup is hiring "the best people" and all teams are "A+". Under the current, debased meaning of the word, it's trivial to be excellent at both.

But, to me, to achieve excellence requires becoming the embodiment of the craft. An excellent engineer, when they have a spare moment of the day, they're thinking about an engineering problem; when they encounter a problem, they analyze it like an engineer; when they cook dinner, they cook as an engineer would. They don't just merely work as an engineer, they live as an engineer.

And because of this, you cannot serve two masters. Once you become excellent at engineering, you can't become excellent at design because to get to an level of excellence would require doing things which your engineering brain is screaming at you are wrong to do.

What is possible, is to become excellent as a designer/developer which is, instead, to focus on the embodiment of the inter-relationship between the two. To look at all problems as a fundamental tension between the two world views and an appreciation of how to thread the middle path.