I've played a bit with VirtualBox. Which version of VirtualBox are you using? The reason I ask is that as of VirtualBox 3..2.4 multiple monitor support is officially only available for Windows guest OSs.
That said, according to what I've read it is possible to hack a bit to get full screen support. You need to install something called "Guest additions". This link might help:
http://www.unixmen.com/linux-tutoria...-ubuntu-fedora
Make sure to install the graphics card drivers for Linux. Once Guest Additions is installed go into System -->Preferences -->Displays. There is an option there to "Detect Monitors". It might allow you to see both your monitors and choose the right one. You can also try playing with your /etc/X11/xorg.conf file settings in Linux.
Good luck!