In this case, you will need to leave your XP drive connected when installing Win 7. Start the win 7 installation and tell it to install on the other drive. You will then have a dual boot menu when starting your PC.
If both OS's are already installed, then you will need to do a clean install of win 7. I'd wipe the win7 drive before starting otherwise you may end up with a boot menu that contains XP and two versions of win 7.
Instead of a clean re-install of 7, you can use EasyBcd, a very nice piece of freeware, to make your Win7 boot sequence aware of the presence of the XP installation. This way, when you boot off the Win7 drive, you will be presented with a choice.
And there are quite a few boot managers that deal with the issue before you get to any bytes installed by Microsoft, but I don't have experience to help you with them.