You have to extend the desktop to activate both monitors.
The easiest way to do this is by going into the Display Properties/Settings tab, click on the monitor that is shown in greyed over color. When you do this, the option to check the box to "'Extend my Windows desktop onto this monitor" will be able to be activated by clicking it's checkbox.
You can also install the ATi Hydravision tools that ship with your drivers, it will walk you through a wizzard that accomplishes the same thing (except hydravision has a lot fo other functionality as well)
i did the "extend my desktop" way and the monitor icon marked second was initially highlighted but right after i hit "apply", no highlight anymore and no signal to the second monitor.