If you're using separate HD's then always unplug the other HD during OS install, then boot your choice by setting preferred HD to boot first, using the one-time BIOS Boot Menu key to boot the other.
If you don't like this arrangement you can install EasyBCD to Windows 7 to add XP to a Dual Boot menu on the Add OS tab. This will still keep each OS independent to boot itself via the BIOS.
If you've already installed XP to the other HD with Windows 7 plugged in, then restart Windows 7 by marking it or it's 100mb System Reserved partition Active, then run Startup Repair - Run up to 3 Separate Times.
Then you can install EasyBCD to Windows 7 to add XP which always works best.