You can not easily do a dual boot using Win 7 after Win 8 is installed. Win 7 does not recognize win 8. You need to repartition the drive, install Win 7 first, them Win 8. I had seen years ago a procedure for doing XP or Vista after Win 7, but don't have the link. You could search the web for some help here.
I don't believe Amogh is correct. Usually UEFI needs to be disabled to install optional operating systems.