Pretty sure Windows is going to need its own partition. Windows doesn't like to share. Plus, the filesystems for Windows and Macintosh are incompatible.
A better question would be "which one do you install first?" Install OS9 and build the Windows partition, and then install Windows? Or install Windows and build the OS9 partition? It all depends on which OS is better able to deal with being installed on a non-primary partition. Linux distros come with partition managers, but I don't think Windows or OS9 do.
An even better question: how do you double boot? Do you use something like grub?
Very interesting project, I hope you have stories for us before you accidentally fry everything.