Did you download the Win8 ISO, burn to DVD using Windows Image Burner, then boot the installer to install to the correct HD? If not do that now. It should configure the Dual Boot.
If you want to keep the HD's independent you can unplug Windows 7 during install to Win8 HD, then after install set preferred HD to boot first in BIOS set, boot the other HD using one-time BIOS Boot Menu.