If you simply connect both hard drives, one installation of Windows will not automatically detect the other and give you an option of which to boot. Therefore you have two options:

1.) Let your system BIOS handle the dual-booting. It may have a menu letting you choose which hard drive to boot from. If not, you will have to change the BIOS boot order from one drive to the other every time you wish to boot the other OS.

2.) You can boot to the OS on whichever hard drive is the primary boot drive, install VistaBootPRO, and use it to create a boot menu containing an entry for both OSes. (Preferably your Vista drive would be primary so that you do not have to force-add the Vista Boot Manager, just add an XP entry to its existing BCD store.)