It should not be necessary to disconnect/disable a hard disk in order to install Linux. If you are installing a well-behaved Linux distro, it will detect both disks, and ask you which one you want to install Linux on. It should also offer you the opportunity to partition the disk to you liking, with as many partitions as you want.
For example, I use SuSE 8.2 Pro, which showed me both of my disks, what partitions I have, what OS's are already installed, etc., and offered me the opportunity to make and format partitions to my liking.
You should be able to make a partition for xp to use, but you may not be able to format it until you boot into xp. I'm not sure since I don't use xp.
For Linux, you need one swap partition (minimum size equal to the amount of ram you have). It will be used by all Linux distros you install (since only one is booted at any given time).
You will need one or more partitions formatted for Linux, depending on the partition structure you want.
Monolithic - one partition for the entire Linux installation. As many as the number of distros you want to install.
Or, many smaller partitions, so that you can spread each distro over several partitions, such as:
each distro has it's own / (root filesystem) partition.
each distro has it's own /boot partition.
each distro has it's own /home partition.
and so own.
If you install the Linux bootloader to the MBR, you won't be able to boot xp without a boot disk, until you have the bootloader config setup. If you don't install the Linux bootloader to the MBR, you will need a bootdisk for each Linux distro. Easiest way - install the bootloader to the MBR, then search these boards for 'dual boot xp' issues. There are tons. Editing the bootloader config file to make the necessary entries to be able to boot both Linux and xp is easy and straightforward.