|You don't merge hard drives to create a multiboot. You are asking the wrong question.|
It would appear you have installed each drive as a standalone when you installed the OS on each.
You can not make a multiboot in this situation except by chosing which drive in the bios you want to boot.
To create a multiboot you need to designate one drive as primary and then reinstall the OS or do a repair install of the 2nd OS to correct the registry pointers as well as create the multiboot.
Here is a how to if you set the xp drive as primary
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