You can easily add multiple ext partitions to your disk, then boot a live-CD with whatever linux version you want on it (unetbootin to easily install live-Cd to usb), then when you boot into that live system to install, just choose the new partition you just made for / (root). You can use a separate /home partition for sharing large data like music,files, videos between installations, and then add multiple smaller root partitions with completely separate operating systems on them (12.04/13.10 etc) so that if you want to get rid of one or replace one with a different version or operating system altogether, your personal files don't get deleted because they are on a separate partition.