There is no concern in dual booting Windows 8 with Windows 7. Just boot up with the Windows 8 DVD, select the extra HDD in which you will install the Win 8 OS and you're good to go. In this case, you're actually not dual booting, you are just installing a different OS on a different hard drive. So you will need to change the boot order in BIOS to boot to the required OS.
Or another option is to shrink the size of C drive and create another partition out of it. Then install Windows 8 on the newly created partition. This is dual boot in true sense.