After getting a Mac, how to install Lion again on another partition?
I usually just use a disk cloning app like Carbon Copy Cloner to duplicate my existing Lion install to the other partition. Then you can boot into your new partition and install the Xcode beta.

This method is nice because it moves all of your documents and settings along with the OS so you don't have to use Migration Assistant after installing on the new partition. It's also convenient if you don't have the original Lion installer.