Who starts, and who will be the backups will be determined in training camps.  In todays pass happy NFL, there is plenty of room for multiple DB's on a roster.  Baalke and the coaching staff have shown that they know what they are doing by virtue of taking the team to the NFC Championship and the Superbowl in their two years in San Francisco, so I have to believe that their decision to bring in Nnamdi is a wise one.