Okay, here's how you do it;
Install one of them.
Start menu > right click my computer > manage > Storage > Disk Management > right click that partition and click shrink volume.
Try to give both partitions equal size
If you're formatting because you've backed all your data up, just delete all partitions on your drive from the Windows 7 disk, then create two equally sized partitions, and install to one, then the other.