Because of the way the iPad's operating system, iOS, works it is not possible for one app to 'lock' another in the way that you want. An app can password protect its own files or 'lock' them, but it cannot do this to another apps's files. Apple did this to isolate or 'sandbox' apps from one another in order to prevent any possibility of virus infection since any virus would be unable to infect or affect any other app.
Tim