Your problem may be that your total horizontal resolution may not fit within the video card's maximum display area. Use 'xvinfo' to list the video card's attributes. Look for the 'maximum XvImage size:' line. For my 8500 Nvidia card, the max size is 2046 x 2046, so 2046 is my max horizontal resolution. (I only have one monitor, at 1900x1200) You didn't mention the res of the second monitor, but I assume it is 1280 horizontal as well. That means your card must support at least 1280+1280=2560 max horizontal resolution. If xvinfo reports max XvImage as less than 2560x2560, the blank edge is outside the max display size. I suspect the max size will be 2456x2456 (as 1280 + 1176 = 2456).
If this is the case, I don't know if there is anything that can be done to change the max display. I have never had any luck changing the size or aspect of the max display area. I believe it is determined by the hardware itself, but I could be wrong.
You can also use the xrandr program (enter: 'man xrandr' for details) to adjust your video outputs on the fly, maybe it will help get a display setup you are satisfied with.