Yep, definitely sounds like you need to set your dualies to Horizontal Span mode... if you're running an NVIDIA card:
To do this right-click your desktop, select Properties then Settings, and press the Advanced button.
When the dialog box pops up click on the tab for your NVIDIA card, select nView display mode in the sidebar and in the nView modes field choose "Horizontal Span".
That should fix you up!
Here's what I'm talking about (sorry for the wide image)
That'll do donkey... that'll do...