How do I dual boot Windows 7 and OS X Mountain Lion on two separate drives?
Windows on internal, OS X on external

Use Internet Recovery (boot with ⌘⌥R) to obtain an install media, then select your external disk as the install destination.

You can do a normal ⌥-boot and select your external disk as the boot drive, which will boot into OS X.



OS X on internal, Windows on external

You can't use Boot Camp with an external disk, per the official user guide:


  
  Your Mac’s disk must be an internal disk. You cannot use Boot Camp Assistant to install Windows on an external disk.
  


Source: http://manuals.info.apple.com/MANUALS/1000/MA1583/en_US/boot_camp_install-setup_10.7.pdf

However, you might succeed by creating a Boot Camp partition on the internal disk, then cloning the partition to an external drive. Once the clone is done, you can delete the Boot Camp partition from the internal disk and extend your OS X partition.