Your videocard may have special software (for example, Nvidia Control Panel) that will properly identify the displays and allow you to run a wizard to set up all the options you want for dual displays.
I can't get mine working properly because as soon as I turn off one monitor to go away from the computer, it thinks the monitor is gone forever and shunts the desktop and everything to the second monitor as I'm about to shut that one off, so when I come back and turn them both on, I have to fix my entire desktop so I decided it wasn't worth it.
Hopefully things go much smoother for you than they did for me.