Is it possible to dedicate a monitor for VirtualBox use?
This cannot be done as Virtual Box is an application that runs within Windows. You will have to extend the desktop to the other screen. You can however maximize the Virtual Machine to that screen size and use it solely for the VM.
I have found however that there may be a way that you can "dual-boot" two operating systems as the same time using andLinux. I found this article about it. I have not used this software and do now know how it works, but this may be a solution for you.