The second is this unit: www.matrox.com/graphics/en/products/gxm/dh2go
You would have to set the monitors to the same resolution - for the sake of argument let's say they're both 1920x1080. The Matrox unit then tricks the computer into thinking that you're using one 3840x1080 monitor and then sends the right half of the picture to one monitor and the left half to the other. This is closer to what you want, but instead of having one "main" monitor and one "extension" both monitors would be the "main" monitor together and the taskbar would be split across both.
