Yeah, just an idea. For example programming the GUI(2d) is displayed with 1024x768(predefined), while the 3d part is in 1600x1200/800x600,...(changeable at runtime)
Perhaps, this is much too difficult to code this feature, but anyway it would help many users.

Edit: Well, physically the monitor can't display different resolutions at the same time. But perhaps, the engine can simulate the 2d and 3d part.(3d the same,2d scaled)