I disagree with this premise. This is like someone saying that a person could be a world class brain surgeon and a word class psychiatrist at the same time. True there are a lot of things in common - You have to go to medical school, you have to know about the brain, etc. However, how you relate to patients, the culture, the tools you use are totally different.

I think it is the same with developer and designer. A talented person would be able to do both. However, you are not going to be able to do both on a world class basis. At least for developers, it seems the top developers are 10x better than an average developer. There may be something similar for designers, I don't know. Hiring a world class developer and a world class designer is going to make better sense than hiring 2 people who do a bit of both.

It seems everything is becoming more specialized. In IT, we are now at the point where you can't be a "generalist" developer-designer but need to specialize in one or the other to be productive.