Well for one thing the next version of OS X hasn't been announced yet..
And as to how to keep the two installation separate..I imagine this is what is happening..
You created two installations of OS X and in each you create just a single user, one on your installation and one for your brother on his. Now OS X being a Unix-based operating system uses UID (User ID) and GID (Group ID) to determine access permissions to files/directories..
So if open up a Terminal window and type in 'id', you will see:
Code:
uid=501(ashwin) gid=20(staff) groups=20(staff),402(com.apple.sharepoint.group.1),12(everyone),33(_appstore),61(localaccounts),79(_appserverusr),80(admin),81(_appserveradm),98(_lpadmin),100(_lpoperator),204(_developer),401(com.apple.access_screensharing)
Note that my UID is 501, and GID is 20. If you run this command on both of your installations, my guess is that you will get back 501 and 20 as the two values though the usernames might be different.
Technically, OS X doesn't care about the usernames, just the UID/GID, so since they likely match, it assumes either account on the either installation of OS X can access the other. If the UID were different, then you would still be able to see a lot of folders in either installation, including the home directory under /Users/<username> and the only way to change that is to reset the directory permissions.
Right now, my home directory is:
Code:
drwxr-xr-x+ 84 ashwin staff 2856 Apr 10 09:30 ashwin
The drwxr-xr-x means, [d]irectory, [r]ead, [w]rite and [e]xecute for owner (me), read and execute for group (staff) and read and execute for other (everyone else).
If I wanted to restrict access just to myself, I'd change the permissions with
[CODE
chmod 700 /Users/ashwin
[/CODE]
This would change the permissions to:
Code:
drwx------+ 84 ashwin staff 2856 Apr 10 09:30 ashwin
Now you'd have to do something like this for every folder that you didn't want the other installation to see and so on..and all of this hinges on the fact that you have to ensure the two user accounts have different UIDs..
A bit complicated, eh?