To merge patients into one family account, you will need to go on the Server computer and select Managers > Database Utilities > Move Patient to Another Account. Select the top magnifying glass and search for one of the patient's name. Select the bottom magnifying glass and search for the next patient's name. Click the go button and MacPractice will prompt you to logout of the client computers and to save a backup.
This will move both patients and all their files into one account.