That's a very limited view on the capabilities of both types. And overly discouraging. "Lifetimes" worth of study are not required to be more than competent in both fields, otherwise no one would get hired for professional art OR code until they were in their late 50's. And the rest of your post just reads like "I tried and failed, so it's not possible."
They (well, Malcolm Gladwell) says you need to spend about 10,000 hours doing something before you're an expert at it. Based on that rule I'm an expert in at least 2-3 things so far, and I'm only 32
There is definitely value in sticking in with something until you get good at it, mind you, but if you happen to be passionate about more than one thing then why not pursue them? It needs to be for the "right" reasons though, or it'll never "stick".
Edit: more directly related to the thread, here's a great blog post by Tommy Refenes (of Super Meat Boy fame):