If your guest OS is Windows 2012 and you have installed the Virtual box guest additions for Virtual box version 4.2.16, then you can expect a crash or freeze of your guest OS. This is due to Windows 8 video driver which comes as a part of Virtual box guest addition.
To resolve this issue open the device manager and uninstall “VirtualBox Graphics Adapter for Windows 8” from the "guest OS". This roles back the graphics driver to default Microsoft display driver. Shut down the guest machine and change the monitor count to 2 in settings of Windows 2012 guest OS and un-check 2D and 3D video acceleration. Start the guest VM, that's it, your guest OS will now support 2 monitors.
Device manager Link in Control Panel
Virtual Box Settings
BTW, if you look at the device manager again, this would still point to “VirtualBox Graphics Adapter for Windows 8” as we have not removed it in Safe mode. But this will work and its working for me.