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#130
I did as you instructed. Shrunk the Windows 10 drive and booted Windows 7 install from USB. Selected to install on the blank space and that was it - it worked. Boot menu looked like attached photo (sorry about the quality).
Now I have to figure out how to REMOVE Windows 7 without messing up Windows 10 boot record. I followed this guide: Dual Boot - Delete a OS - Windows 7 Help Forums
However, I must have been going a little fast because I realize JUST NOW that I missed step one.
Removing Win 7 resulted in boot failure in Windows 10 so I had to re-install Windows 10.
Bottom line, however, is that you CAN dual boot Windows 10 and Windows 7 with Windows 10 installed first.