Dual-boot Repair tool can help in this situation.
Let the computer boot not selecting or typing anything when booting so first disk in BIOS is default for boot.
In whatever system comes up (Win 7 or 8) download and install the tool.
Click on "Automatic Repair" read what it displays and confirm.
(It should say that Win 7 and Win 8 are found and Win 8 will be default OS to boot after repair.)
After successful repair when you reboot you will have either Win 8 and Win 7 in boot menu
or just Win 8 will boot directly.
If Win 7 is not in boot menu - install Visual BCD (if it is not already installed) and run
Visual BCD Editor.
Right click in left pane and select "Create missing Windows loaders".
This will create a boot menu entry for Win 7.
Renaming entries - click to select loader entry in left pane then F2 key and change text.
Hope this helps.