What you should do is what you want to do, and if you enjoy both topics, there is no reason to pigeonhole yourself into one of them. It is possible to be successful by focusing on breadth as well as depth, although the challenges are different. You are the only one who can decide what is right for yourself and for the kinds of jobs you want to pursue.

As far as whether it's possible to be excellent at both design and development, the answer is clearly Yes -- if you have 10,000 hours to put into each of them. It's pretty hard to find a role where you can focus on two distinct areas for 10 years, but it's possible (though often when this happens, one is professional and the other is a hobby).

However, you should recognize that both design and development are very fast-moving areas right now, and so being an "expert" in one of them usually means you are good at problem-solving and you can search online competently. So there may be a lot more overlap than some people think, which would make it easier to be an expert in both.

If you are interested in management theory, the traditional view is that there is a tradeoff between specialization and productivity (think Scientific Management, Division of Labour, etc.). A more modern perspective is that jobs that require creativity benefit from job variety and task identity (basically the ability to work on more aspects of the problem rather than just being one small cog in a large machine).