If you want to dual boot the Microsoft way install XP to one drive and then select the other drive during the Vista setup. The vista boot loader will be installed to the XP drive and you will get a boot menu when you boot up the PC. You just pick which OS you want. The downside is if you format or remove the XP drive Vista can't boot up. Also removing the Vista boot loader from the XP drive can be a pain.
If you want them totally independent disconnect one drive and install XP. Then unplug that drive and connect the other back up. Now install Vista to the second drive. Power down again and connect both drives. Your Bios boot order will determine which drive the PC boots from and determine which OS loads up. The downside to this is if your bios doesn't have quick access to the boot menu switching OS's could be a pain.
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