|No need to dual-boot. Use single-boot method which I have been using for some time. The procedure for single-boot is delineated below:|
You already have W8. Disconnect the HDD/SSD on which you have W8. Disconnect other HDDs/SSDs, if you have them for data storage, backups, etc.
Connect a blank HDD/SSD on which you want to install W7. Install W7 normally.
Now re-connect the first HDD/SSD with W8. Re-connect the other HDDs/SSDs for data storage. So you have at least two HDDs/SSDs connected, one with W7 and the other with W8.
Whenever you restart your PC, you will boot directly to W8. If you want to boot to W7, then you have to press a function key, such as F8 or esc or whatever, when you are at the BIOS post screen. This will give you the option to choose which drive to boot to. The options you have are the HDD/SSD with W8, the HDD/SSD with W7, DVD drive, removable drive such as USB flash disk. You can navigate between bootable drives with the keyboard up/down arrow keys.
You can choose which one you wish to boot to without affecting the other drives. You can add as many drives as your motherboard SATA ports allow with different OSes on them. W7, WXP, W8, Vista, Ubuntu, Linux Mint, etc.
My motherboard has 6 SATA ports. I can connect 6 SATA drives and install 6 different OSes individually.
Hope this helps.
Caution: My Asus P7P55D motherboard allows me to choose during boot which drive to boot to. Check if yours also does the same.