Just wanted to add that I am experiencing somewhat the same behavior for a Windows 7 Enterprise 64bit guest/host (both same OS) machine/vm.
Minimizing the VM seems to cause it to lose the secondary monitor as well as locking the host for me. Also going to another VM WS tab can do the trick.
It happens all the time so it is difficult to say what exactly can cause this.
As for launching applications that require elevated priviledges, I don't experience that they cause it to lose the second monitor as seems to be what is described here. Instead it seems to happen randomly sometimes, when I am in the middle of something the one monitor suddenly disconnects, this is very rare though where as the 2 above scenarios are much more frequent...
Locking host causes this like 80-90% of the times.
Minimizing causes this like 50% of the times.
Navigating to a different VM tab does it 20% of the times.
(These are rough estimates of frequency)...