In the virtual machine settings (gear icon), under the Options tab's Full Screen section, try checking "Use all displays in full sceen."
The problem that I am having is that while multiple monitors are used, the mouse click are not correctly translated. The GUI is completely usesless. All I can do is to right-click somewhere to get the option of opening a terminal window where I can type something. Looks like a Parallles Tools bug to me. This happens at least with Ubuntu and Fedora.
