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

Excellence in both is rare. Shaun Inman is a remarkable example.

Imagine you go the coder route. When you going to specialise in there. Big data? Databases? Embedded coding? 3D graphics? Game coding? Web app coding? Front-end? Back-end? Testing? People can make careers out of any of them - and many more besides. It's pretty much impossible to be an expert in everything on the coder side.

Imagine you go the designer route. What are you going to specialise in there? User testing? Icon design? Branding? Web design? Print design? Interaction design? Information Architecture? People can make careers out of any of them - and many more besides. It's pretty much impossible to be an expert in everything on the design side.

I've had a pretty successful career with a bunch of things from the developer side and a bunch of things from the designer/UX side.... and some other bits and bobs from other things like project management and business folk.

I'm really good at interaction design. I'm really lousy at visual design. I'm really good at testing. I have no embedded coding skills. I'm great at... well... you get the idea...

Design and Development aren't "things". They're communities of practice. They're social constructs. New ones appear and disappear over time.

Pick "front-end developer" for example. Twenty years ago that role didn't exist. A bit of layout knowledge. I bit of a design eye. A bit of development. A bit of markup. And so on. Those skills existed - but there wasn't a 'role' where you needed to be expert in all of them.

In another ten or twenty years I'll be surprised if anything like "front end developer" exists in the same way (look at how folk using stuff like node.js blurs the coding side of front/back end development for example).

Figure out what you want to do. Then acquire the skills you need to do that - and be excellent at those. The choice isn't between being a design expert and a development expert. Instead make a choice to be excellent at the things you enjoy and are good at - and then go find career where you can use those skills.

Time is your main resource and nobody can do it all at the same time. Some things take a lot of time just starting to work and if you change from one thing to another during the day you don't do anything.

But don't believe me. Test it yourself. Measure your time, take your smartphone timer and monitor yourself along the day, write it down how you spend your time, not as you should, but as it really is.

If you are as good as you said you can pick one as your main area and them choose someone who knows how to do the other. Having "taste" and knowing who to choose is not as easy as it sounds.

(*) Yes, there are examples of generalist who have led successful startups, most of them are done by generalists, if for generalists you mean someone who is able to trust others doing the specifics because they know they can't do it all. With a startup you have to be accountant, tax payer, product evangelist, sales manager, marketing head, engineering head all yourself...

It's actually "worse" than that. It's pretty impossible to be expert in one field.

I don't know anybody who is expert in, for example, embedded development and game development and back-end web development.

I don't know anybody who is expert in, for example, font design and user testing and icon design.

Yet developers and designers can be successful by picking subsets of "design" and "development" to be expert in.

There is nothing stopping folk being expert in subsets of "design" and "development" - apart from the notion some people seem to have that one skill set interferes with the other.