Do you know if you are booting via UEFI or legacy-BIOS? You can check pretty quickly by starting up "disk management" (just type that in the start screen and it'll pop up in settings as "Create and format hard disk partitions") and checking whether your hard drive is formatted as MBR or GPT. Right click your drive in the lower list (it will probably be disk 0) and click properties, then check the "Volumes" tab, it will list a "Partition style": MBR means you are booting with BIOS, GPT means you are booting with UEFI. (At least, it does in Windows).
If it's via UEFI, Windows 7 supports booting but isn't quite as compatible so you may have to make sure things like Secure Boot are disabled.When you install Windows 7, boot from the DVD (or USB?) and select "custom installation" and you should be able to choose your second partition as an install location (select it, click next). This will set up Windows 7 as the default (and only) boot option, so if you are using UEFI you must also have a Windows 8 installation DVD or preinstall environment so you can install the Windows 8 bootmgr again.
If you are using BIOS you can just use the windows 7 bootmgr and add a windows 8 entry to the bcd, use something like easybcd or visual bcd editor, it's fairly straightforward. If you do have to reinstall the windows 8 bootmgr, rebuild the bcd in the recovery environment (look up how to repair a damaged bcd, it's the same steps: basically cd C:\boot, attrib -s -h -r bcd, ren bcd bcd_old, bootrec /rebuildbcd, which should scan and find the Windows 7 and Windows 8 installations and add them to your boot options).
IMHO even if you are using BIOS the windows 8 bootmgr is a little slicker so you'll probably just want to use it regardless.