Apple TV, content from two users on same mac
Firstly, in order for AppleTV to see the iTunes Library a user must be logged in and running iTunes. If both of these are not present then AppleTV won't work.

Since they are on the same physical Mac might I recommend combining the iTunes Media folder and placing it in the /Users/Shared/ directory. As all users have access to the Shared directory the actual media files wouldn't be duplicated and they can each maintain their own iTunes Library. After moving the iTunes Media folder to the Shared folder you will need to go into iTunes Preferences for each user and under the Advanced tab select the new iTunes Media folder location.