To use a HDD of that size you MUST use the GPT Partition system. Considering what you plan, that means you will need to Partition into those two partitions, making the first one the 1 TB Partition and ensuring that it is bootable. THIS step can be done for you by the Win XP Install process (below). After that is done, create the second Partition using all the remaining space, but this one does not need to be bootable.
To do this you must have two things.
1. Your computer's BIOS must have UEFI Support. This is the tool that allows your BIOS to find and use the boot files on a HDD with a GPT Partition.
2. Then you must install a 64-bit version of Windows XP on the HDD unit. Only the 64-bit version of Windows XP includes a "built in" device driver to use GPT disks, and that is going to be needed for it to BOOT from a GPT Partition.
If you have those things available, then I suggest the following sequence.
(a) Connect only the 6 TB drive unit. Go into your BIOS Setup and ensure it is configured to use the UEFI Support system to access the drive.
(b) Still with only the 6 TB unit (not the SSD) connected, run the Windows XP 64-bit Install process. Use it to create a 1TB Partition and install to that. When finished, use disk Management to Create a New Simple Volume out of all the remaining HDD space.
(c) Shut down and connect the SSD. Use the Windows 7 Install process to install that to the SSD, making sure that it sets up a Dual Boot option that makes the existing Win XP OS on the HDD available.
In earlier versions of Windows, setting up a dual boot system only worked if the new OS being installed was more recent than the other one. So Win XP would have to exist and be accessible before the Win 7 version could be installed that way. There did not seem to be a way to add an older OS version to an existing (say, Win 7) OS and create the dual boot option. That is why I suggested this sequence. HOWEVER, I and NOT sure that rule still applies to your particular situation, so try to check that. I say that because it appears from your post that you already have Win 7 installed on your SSD, and would be reluctant to do that over again.