You can use the same DM for both devices. The easiest way I have found to tell the difference in backups is to add my wife's name to the backup file from her BB when saving it. That way I have my none edited back-up and hers. The only issue I ever had was when syncing with outlook. She ended up with all my contacts because I forgot to set it not to sync. I am sure there is a way around this but someone else will have to explain it.