Using a single user account between two mac OS installations on same machine
I accidentally set this is up and it runs perfectly. I have mountain lion to run old things I still need and Sierra on a separate partition. They both have full administrator access to each other under my user ID. 

The way it happened was I installed Sierra and then copied the data from my mountain lion installation. From the new Sierra I had full access to the mountain lion partition including system folders and vice versa. I can even run some apps like VLC and Office from the other OS’s apps folder. 

Now I want to reinstall Sierra but when I copy the data from the old Sierra partition it no longer recognises the two user IDs as the same me and I can’t access the other OS from either. 

I would love to know the logic governing what makes two IDs recognisable as the same user so as to find a way to reinstall Sierra with its current state AND retain access between the two OSs.