If you have a bootable XP installation installed on the one drive, have both drives installed in the system at the time that you install Windows 7, and select a custom install and tell it to install to the appropriate partition on the second drive (just make sure you choose the option to tell it where to install), it will automatically create a dual boot that defaults to Windows 7, but will leave your XP installation available as the option "Previous Version of Windows" on the boot selection screen.
Just be extremely careful about which partition you install 7 to - make sure it's not to the drive (in actuality, it's the partition that matters but let's assume that you have only a single partition on each drive) that has XP already installed.
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