dual boot Mountain Lion/Mountain Lion
You WILL need to reformat the smaller partition from NTFS to HFS+ before booting to the installer but otherwise it's pretty straight forward.  When you boot to the installer / recovery partition you will eventually be prompted to select the installation destination, choosing the smaller partition will tell the installer to install 10.8 on that partition rather than your main partition.

Once the installation is complete you can choose which one to boot into either by specifying it in System Preferences > Startup Disk or by holding option at startup and selecting the proper partition.