Two operating systems running at the same time in one computer. Is that possible?
Yes, running more than one operating system (OS) in one machine is possible. When you're running Windows and want to try using a Linux distribution at the same time, that is possible. And it can be the other way around. The same goes in situations where one is using Mac.
That is done through the virtualization (or virtual machine) technology. That means installing a third-party software (e.g., VMWare, Virtualbox, among others) and use such software to install another OS of your choice, which is called a guest OS.
If you're using Windows, you may install Linux using the WUBI (Windows Ubuntu Installer) approach. Visit http://wubi-installer.org.
If you're using Linux but want to use Windows for various reasons, we suggest that you try Virtualbox. You may try this Techthrob link on how to do it.