The text book way of dual booting Windows operating systems is to install them in ascending order of versions. For example in your case, Windows 2000 first, then Windows XP second. This is because Windows XP is capable of detecting older versions of Windows, and configuring the boot record to accommodate them as entries for you to choose which OS to boot into.
Windows 2000 won't detect XP. If you install Windows 2000 after having installed Windows XP, then the dual boot won't be configured properly....
...However, you can still install Windows 2000 after XP, and have the dual boot system functioning correctly. All you need to do is, after having installed Windows 2000, place the Windows XP Setup Disc in the drive, boot into the CD, enter the recovery console, log into the Windows XP partition, and then re-write the master boot record (with FIXMBR) and re-write the boot configuration file (with BOOTCFG) - thereby forcing Windows XP to detect all operating systems and configure everything for you, to allow the ability to select which OS to boot into.
Also, on the matter of installing operating systems on independent hard drives, so long as you've set the drive's jumpers correctly (hard drive hooked up to master IDE set to master) then as said, you won't need to move the jumpers around to boot from a specific OS...
...Whether you install Windows 2000 first then Windows XP, or if you install Windows 2000 after XP and re-write the Master Boot Record and boot.ini file in recovery console, Windows XP is capable of detecting operating systems on all hard drives connected to the system, and creating entries for them automatically for a dual boot config.
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