Boot from the Mac installer DVD and after selecting English stop there.
Under the Utilities menu, select Disk Utility.
In DU, select the actual device (not the Mac OS volume currently installed) and click the Partitions tab.
At the bottom of the Window, click the plus (+) sign. This will add another partition. Size it accordingly. Name the partition (no spaces or special characters and different from the existing partition).
Select (from the drop down) MacOS Extended (journaled) and click Apply.
Then you can install another MacOS on the 2nd partition.