There are people who are highly focused, and extremely good at what they are focused on. There are also people who excel and moving between and integrating fields and styles of work, and they are very good at forming a cohesive whole.

Genius is achieved when these people work together seamlessly.

I think one can do both sides unless they have mental or physical defects.

I think this is a bit unsensitive to be honest. I have no clue how you interpret it, but personally I read: "If you can't do both, you're either incapable due to a physical handicap OR you're nor right in the head." and that there is no other reason one is not both artistic and technical. I'm going to assume you simply worded it in a way that made me interpret it wrongly, but I suggest that you keep in mind that people can find this offensive and think you might be a tad narrowminded and/or elitist because you claim to be both. I'm not saying this can't be a reason, but don't claim it to be the only reason.

I also realise this is your opinion and I don't mean to criticise you in a negative manner, just wanted to make you aware people can interpret it wrongly as not everyone gives the benefit of the doubt.

That being said, more to being on topic:

What separates artistic and technical people is a factor of many. It can be a lack of interest in one or the other or the other way around, more interested in one than the other or what you already suggested. Everyone has their own reason for not having an interest in one or the other.

*edit some typos and clarified some more.

I think this is a bit unsensitive to be honest. I have no clue how you interpret it, but personally I read: "If you can't do both, you're either incapable due to a physical handicap OR you're nor right in the head."

That's not how I took it (and when reading any forum, or critiquing in any medium, I beleive you should always default to the most favourable interpretation); I assumed that he means that anyone can learn to do anything, unless of course there is some kind of physical barrier to learning that task.

e.g. That if someone is "logically minded", that's simply a matter of their upbringing/training so far, and they could be taught to think/see in other modes if given the right learning situation (it's well known that people of different backgrounds actually think about and see the world in different ways, using different parts of their brain, and that this different modes of vision/thought can be learned). Something like a stroke may interfere with this, as it physically kills off certain areas of the brain.

We all might find learning certain things more/less difficult than others, and may need to learn them in different ways. Given the right situation though, you can be taught.

Now, there are technical artists, and artistic artists. There are technical programmers and artistic programmers. I lean more towards artistic in both cases.

This is kind of off-topic from your intended discussion point -- but in the industry, a "technical artist" is fairly well defined as someone who can use all the tools that an artist uses (like Maya and Photoshop) and can produce art with them, but who's job is to actually build extensions, sub-tools, scripts, importers, exporters, and pipelines for those tools, to help integrate the team's art assets with the game engine, and to solve problems that come up during art production. They apply their technical skills in order to make the art-artists more productive or more able to do their job.

What makes a good graphics programmer then? Do they have to understand art?

Graphics programming is one of the most processor-intensive areas of programming, at least if going for photorealism. At 1080p, there's over 2 million pixels that need to be calculated -- no other area of a game will routinely deal with data-sets of that kind of size. This means that a great graphics programmer must be well schooled in basics of computer architecture and computer science, and know many low-level programming tricks so they can write fast code.
They also have to have at least a casual knowledge of how art is produced -- they might not have to be able to create great art themselves, but they must be familiar with the tools and techniques that their artist colleagues are using. They need to be able to communicate effectively with these artists, as they are basically a supplier of technology with these artists as their client. Without being able to speak their language, then the specifications/requirements will be all wrong.
They've got to be curious about the world and have a technical eye for detail, but must also have an eye for art -- when viewing a beautiful sunset in real life, they must be able to critique what makes it beautiful (think colour palettes, framing, composition, etc) but also be wondering about what the underlying physical and biological processes are behind the formation and perception of those colours, and what kind of formulas could be used to model them.

That's not how I took it (and when reading any forum, or critiquing in any medium, I beleive you should always default to the most favourable interpretation); I assumed that he means that anyone can learn to do anything, unless of course there is some kind of physical barrier to learning that task.

I know, and assumed he meant it this way (I choose some bad way of saying it myself as well, edited my responce to clarify it some more), but wanted to make him more aware not everyone can take it that way. It's meanly that he states it as a defect that made me feel the need to point it out.

To add a bit more on the topic:

I started off thinking I wanted to get into the art side of things, this had to do with me trying to be accepted into a game development college that focusses on art and programming (at that time). I had about a year to learn Maya for a part of the intake assignment and build up a bit of a portfolio because I hadn't drawn for a long time (which I always enjoyed). I had everything ready and tried to get into maya/drawing again, but I seemed to lack the interest and couldn't put myself into learning Maya by myself and drawing didn't do that much to me anymore.

This was the point where I decided to dive into (game)programming and I managed to put the creativity I have in my mind in code instead of on paper/screen/etc. I found satisfaction in the simplest of things (hard back then) like making a sphere bounce. It wasn't something mathematical to me, it was artistic. The more I learnt about programming, the more I became aware of the fact that I grew back that artistic part of me, I started to find more and more passion in directly altering the looks in a game mainly by writing shaders that made this possible. Next to shaders, I also enjoy making the eyecandy that isn't directly achieved by shaders, I guess you can see it as a bit of gameplay programming where for example I simply make a flower move with the wind.

I guess what I want to say is that it is also a bit in the eye of the beholder. Where I see art in the code that I write and does its thing on the screen, others see lines of boring code. Where others see a cube. I see how it's lit and see it as a form or art from the programmers side.

I think one can do both sides unless they have mental or physical defects.

I think this is a bit unsensitive to be honest. I have no clue how you interpret it, but personally I read:
*edit some typos and clarified some more.