If you want to install the second OS to another HD, you can boot via the BIOS which is a cleaner method that keeps the HD's independent so they can come and go as you please.
Just unplug the first OS HD when you are installing to the second HD, then set the target HD to boot first, after DVD drive.
After install, replug the other HD. Now when you want to choose the other OS HD to boot, tap the key given on first boot screen for one-time Boot Menu and choose the other HD instead.
If you keep both HD's plugged during install, it will configure a Windows-managed dual boot which means the HD's are interdependent and harder to extricate.