Then unplug the other HD during install, set target HD first to boot in BIOS, trigger the Win7 installer using one-time BIOS Boot menu key given on first screen.
After install, plug back in Win7 HD. Set preferred HD to boot first in BIOS setup, use the one-time BIOS Boot Menu key to boot the other OS HD.
If you don't like this arrangement then install EasyBCD to Win7 to Add XP to a Dual Boot menu.
I removed my original posts from this thread because you ignored them for over a day while only replying to AddRAM so I thought you wanted to work with him.