I don't think extending the task bar is possible with a multiple-card display solution. However, drivers for a single-card multi-display configuration might allow for just that. I know that nVidia does, but I haven't seen a similar option with my Radeon VE at work.
Nonetheless, you *can* do something like moving your QuickLanch part of the taskbar to the second display. In fact, I tend to make taskbar toolbars for the busiest folders in my Start->Programs menu. I just right-click on the taskbar and select "Toolbars..." -> "Add New..." With that option, *any* directory/folder on your system can become its own toolbar on yout taskbar. Once in place and with the taskbar unlocked (IE6+), you should be able to drag any individual toolbar from the taskbar and place it elsewhere on the screen. You can move that individual toolbar to the screen display. Heck, you can even make that toolbar float in the middle of your display. If you move one of these individual toolbars to a screen edge, it will assume a taskbar-like behavior.
You can sorta get a glimpse of what I mean from my old
Samurize screenshot. That snap just happens to be from the right-most display of my main home PC (dual-head nVidia).
-SUO