Does not matter which is instaled first in the method I use. I state method for Doing XP first.
1) disconnect all drives except the HDD that you want to install XP on.
2) Install XP, drivers and programs, verify that it works OK.
3) Disconnect XP HDD and connect SSD
4) Install Windows 7 On SSD, drivers and programs.
5) power down and connect all drives.
6) Power on, go to bios and select the Drive (OS) that you will use most often.
When You want to use the Other OS, simply hit the Hot key during post that brings up a boot menu. For my Gigabyte MB it is F12 and Onmy Asrock MB it is F11. THIS DOES NOT change the boot priority in BIOS.
If Boot priority is set to say Win 7, Unless you hit the Hot key it will auto boot to Win 7 (= Dive C). Your XP (dive D) and Storage HDD (Drive E) will be visible to Windows 7. If you hit the HOT key and select win XP disk the computer will boot to XP and P will now be drive C - Win 7 = Drive D and storage HDD = Drive E. If you reboot the System will revert back to Win 7 UNLess you again hit the Hot key and select XP.
Beauty of this method is NO SOFTWARE bootmenu that can screw-up bot drives.