Since I do not know exactly what you did to reinstall Windows 7, I will just assume you overwrote the BCD store you had originally. The way to normally repair a dual boot situation is to use the bootrec /rebuildBCD command. It will scan for OS entries and add them to the BCD Store. Make sure to boot into the Windows 8 media and go to the command prompt.
When you are done, you might attach a copy of your BCD Store by opening a Administrative Command Prompt window and typing:
bcdedit /enum all > %userprofile%\Desktop\bcdstore.txt
Then either zip and attach, or just attach the text file. You can copy and paste the command in a command window.
You should also be able to use the Windows 8 Install/recovery media to repair the...