I'd say that, yes, those that possess one skill can also develop a second, unrelated skill. Some people might have a natural affinity for both, others will have to work harder. I don't think anyone that wants to become a designer is totally incapable.
More to the point, I think you can learn the techniques and train the eye, but whether you can produce beauty with technique and an eye for design is subjective at best.
I personally am good enough to do commercial art for web apps but I won't be winning any fine art contests, I am fine with that because it is good enough for what I do. When I used to do simulation, I had a friend who was great at 3d art, when I would look at his work and compare it to our work (the rest of the teams) their was no comparison. He had a natural talent that none of us could achieve, to the rest we where good but he was better in a league of his own. He went on to work on the Polar Express and several other films, which in the 3D segment of design that is the major leagues.
The rest of the team (myself included) while good, would have never made it at that level. That being said, many enjoyed long careers in simulation and game development and where not affected by being the best 3D designer possible.
2 Examples:
1. Miche Capone - one of the founders of Photovoo and a mentor at 500 startups. I've worked with him before, fantastic designer. Got a CS degree, never officially trained in design.
2. Jason Putorti - Check out his linkedin profile. He got a Bachelors in CS and Math.
I don't think it's easy, but like anything else, if you practice enough you will get better.
(Note: I don't know Jason but his profile is public so I figured it's ok to link to.)
It seems trite to say, but I believe that it's all about doing it. The most useful thing I've found (and I do this not just with design, but with programming) is to find something I like, and then re-create it. I'll see a shot on dribbble (http://dribbble.com) and decide to create something similar from scratch, by playing around in Photoshop until I get it right (or close). If I get stumped, my last line of defense is to look up a tutorial on the look I'm trying to achieve.
I would never publish anything I've created through this method, but the act of purposeful creation gives me tools and techniques to use down the road.