Its very simple, set up the PC with 2 HDD's.
Disconnect one and install lets say W7 on the one you have connected. Once its all set up shut down the PC and disconnect the W7 HDD and connect the second empty HDD. Now install W8 on the second HDD. Once that's all set up you now have the ability to boot into which ever OS you choose by pressing the boot option key during start up, its F9 on my set up. but you can find out which key is yours by entering set up at start.
I struggled with this for a while and then decided to just use the command prompt. if you currently have a Windows 8 computer and want to add Windows 7 that is already on a separate drive. Install your drive into your computer, boot up into Windows 8. Move your cursor to the top right corner and choose Settings, then at the bottom of the slide out click on Change PC Settings. The screen will change and on the left side choose General, the right side will change, scroll down on the right side to Advanced Startup, click on Restart Now. Then choose Troubleshoot, then choose Advanced Options, then choose Command Prompt. Sign in on the Command Prompt screen. When the command prompt window opens you will type: Bootrec.exe /ScanOS to determine if the system recognizes a windows OS, if so it will list them out and where they are. To add the Windows 7 OS to the Boot Configuration type: Bootrec.exe /RebuildBcd this will scan disks for all installations compatible with Windows and allow the user to choose which to add to the boot configuration store. when completed type Exit in the command prompt and the command prompt window will close and you will have the option on the screen to Use another operating system.