Dual boot Windows 7 & 8 with common data

I have a dual boot system betweek Windows 7 and Windows 8. A third partition is where My Documents is stored. Both OSs point to the same My Documents. But every time I boot into Win 8 it trashes the Windows 7 security on the folder.
Previously I had the same setup but with Windows 7 and Windows XP. No problems. Idea?