If you already had all the information in one exposure, you're not actually increasing the image's dynamic range - merely brightening the shadows and dimming the highlights.
If you already had all the information in one exposure, you don't need more exposures, and you can't increase the captured dynamic range because you already have it all. Full stop.
The reason for doing several shots in HDR imaging is because in most high dynamic range situations, with present digital cameras we cannot capture all the information in one exposure: to get non-blown highlights, shadows become too noisy, or to get well exposed shadows, the highlights get clipped. But if we could, just one shot would be fine. For example a HDR camera like the Fuji Super CCD, or the use of a dense GND filter in a sunset will allow us to capture a high dynamic range in a single shot.
HDR is about capturing all the information (noisefree shadows plus non-clipped highlights) of a high dynamic range scene, and tone mapping it so that it becomes well rendered in the final image. The number of shots needed to achieve it (one, or one million) doesn't play any role in the definition of HDR.
Find here one single RAW file containing the 12 stops of information of a HDR scene. No more files needed because that single RAW file summarizes the content of two RAW files shot 4EV apart, that served well to capture all the dynamic range of the real scene. What you can achieve with that single file, is exactly the same to what you could achieve with the original 2 RAW files, no more, no less.