Not sure how two video cards work, but on my Matrox G400 dual display, I have it set so the desktop basically gets stretched across two monitors. Instead of 1024x768, you get 2048x768. Mouse and programs can be moved across easily. You can also set it up so that the display gets stretched vertically so a 1024x768 desktop becomes 1024x1536.
There are also ways of zooming in on a specific area on monitor one and having it display on monitor two. For example, working in photoshop, you can edit pixel by pixel on the zoom and see how it affects the overall picture in monitor one...
Then there's also the new Edualhead thing that Matrox came out with. Haven't used it much, but for webpages with a lot of vertical scrolling, it gets displayed on both monitors so it reads like a book...