Well, we gave up trying to put everything from olduser TM onto newuser and let Migration Assistant create a "new" user called "olduser" that now has all her music, photos and movies with all the repsective Itunes albums, Iphoto events and imovie projects.
The only issue is that we now have two admin accounts and the account where all the main doecuments and media reside is not the primary account. Since they are both admin accounts, I guess that will not be a problem.
I am still curious as to how you use migration assistant to put your stuff from a TM onto the primary account, but we have a working solution.
If I understand your situation and request correctly.
First of all the two administrator accounts are "equal" in terms of access and privilege use. I would just log-in as the newuser administrator and remove the administrator privileges from the olduser through the System Preferences, Accounts option.
Now as the "only" Administrator you can create folder(s) with shared privileges and/or restrict access to the main the main documents and media, or other folders. Just open the Finder and select the folder and then select Get Info (tap). On the bottom of the window you can set the access privileges.