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.
I'm not really sure if this answers your question