Q: iPhoto will not allow two users on the same computer

I am using iPhoto 9.6.1, with a library that was built using the merge function of iPhoto Library Manager. There are two active user accounts on my iMac, but only one can use iPhoto at a time - even though the other user has Quit iPhoto. So if User A opens iPhoto and receives a "disk locked" message (and iPhoto is not open in User B's account) there is an option to repair permissions. After the repair, iPhoto will open for User A. If User A then Quits iPhoto and User B logs on, User B will receive the "disk locked" message when launching iPhoto. If User B then repairs permissions, iPhoto will open for User B but will not open for User A, even after User B Quits, and so on. I have used Disk Utility to repair permissions. I have tried setting permissions on the photo library directly by selecting "Get info;" that does not work, but viewing the Get info window, you can see that after User A has repaired permissions using the iPhoto utility, User B has then been removed from the permission list, and vice versa. Any help would be greatly appreciated.