Unfortunately this is only supported on rather old video cards. You will need a Radon 7000 or 9250, or maybe some Matrox G200 or G400 boards. All of these are from AGP days, so if you have a machine from the last 10 years and PCI-Express ports, you may not be able to use one of these.
There is ongoing work on the graphics driver but as far as I know none of the new one have multi monitor support yet.