Erm yes, you can have two OS on the same disk. In fact you can have as many as you like really.
When your done installing a second OS - a choicelist will appear when you turn on your computer prompting you to choose OS within a 30 sec. period. If no choice is made it will load your "first" OS.
If the choicelist does not appear do as follows:
Startbutton+Pausebutton --> Advanced System Settings --> Advanced tab --> under "Startup&Recovery" press the "settings" tab --> mark the box "Time to display list of operating systems" and choose a number of seconds.
If you want to install two operating systems on a single hard drive you would partition the drive and install the two operating systems on the separate partitions. If both operating systems are Windows based install the oldest first and the newer version second. You can use virtualization to run operating system within another.