Is it possible to use a 2nd monitor simultaneously with the default one in a PC? If yes, how would it affect CPU usage, memory usage (Physical/Virtual/Pagefile) and what would be the requirements for hardware and/or software?
Yes, you can. You just need a video card that has two outputs. Check the back of your computer and see where your current (default) monitor is plugged in. If you see another socket like it, then go ahead and plug in a second one.
Unless you're using it for gaming, the memory/cpu usage for the second monitor is minimal.
Obviously, you can add a second video card and enable two monitors in the Windows display settings, but if I understand you correctly, you actually one the second display to be an entirely different user logon session than the first monitor, correct?