I am working on a framework for web based apps, including both UX guidelines and the art/graphic design guidelines such as what menus will look like, headers, colors, fonts etc.
The UX designers I met, were unable to provide the artistic side, and the graphic designers didn't have the UX skills.
Should I continue to look for one person with both skills, or is it better broken to two separate tasks?
I would say that the roles are explicitly different. I wouldn't say it's impossible to find a designer/artist, but you're more likely to find a good developer/designer.
The way we currently split UX out is to have developers, artists and what is effectively an Architect/Designer that mediates the two disciplines and has ultimate say over the user experience. I believe some companies call this role a "Program Manager".
As a developer, I've had some trouble with Graphic Designers crossing into UX and driving the project in the wrong direction.
I like to think that graphics are layered onto good UX design. Keeping these two disciplines as separate layers seems like a good idea. You also might benefit from two interpretations from two people.
But I have stumbled on your question because I am still learning (even after 12 years). So my opinion could be a bit unreliable...
It's a great combination as long as the designer can apply an attractive design to a solid UI design, and doesn't let the graphic design drive the user experience. In other words, the designer should understand that an attractive design doesn't necessarily make for a usable interface.
Within the last few years, many "new media" programs have become more sensitive to users' needs and now offer UX design courses; in the past these needs were completely missed. I recall some sites years ago that looked amazing -- like something out of a magazine -- but the pages were anything but usable.
The early stages of user-interface design can be as simple as paper prototypes. No fancy graphics required. Good UX designers must know and use patterns, and at the very least be aware of some UI evaluation techniques (such as heuristic evaluation and cognitive walk-through). Graphics should come later.