Why cant i duel boot windows 8 pro with windows 7 64

i have a  wd  2tb red  hard drive and want to know why cant i partition my hard drive and  dule boot windows 8.1 pro  64 upgrade and windows 7 64. with windows 8.1 pro 64 upgrade  install first thank you Bruce Bergeson

We do not support this type of configurations.

How to install more than one operating system on a hard drive or dual boot a system


You can visit Windows support for more information.