IT isn't carved in stone that a movie has to last about two hours and tell a complete story from beginning to end. Some filmmakers - especially French ones lately - unfold their stories in a more leisurely way, stretching a single plot over more than one movie.Claude Berri did this a few seasons ago with "Jean de Florette" and "Manon of the Spring," based on novels by Marcel Pagnol, a great French filmmaker of old. And now director Yves Robert has given us a pair of movies My Father's Glory" and "My Mother's Castle that can be seen separately, but are really two halves of a single narrative. They are based on Pagnol's memoirs and steeped in the atmosphere of a bygone time for which he and Mr. Robert show great affection. Both parts of the story take place in the early years of this century, and depict Marcel's life as a young boy growing up in Marseille and the surrounding countryside. His family is loving and respectable - his father is a teacher, his mother a seamstress - and also a little eccentric, since the father has somehow become a cheerful atheist who loves to poke fun at his religious friends and relatives. The first movie, "My Father's Glory," focuses on Marcel's relationship with his father, whom he practically worships. When the family goes to Provence for an idyllic vacation, Marcel has a number of small adventures but also enters a crisis when he discovers that his father isn't a perfect being after all. The second film, "My Mother's Castle," finds the family commuting regularly to the countryside they love, and getting into trouble with the authorities when they overstep their welcome. Again the movie centers on family life and the challenges of passing into adolescence, and has touches of the humor that has been a Robert specialty in such earlier movies as "The Tall Blond Man With One Black Shoe" and "Pardon mon affair." "My Father's Glory" opened in the United States a few weeks ago, before its companion film, and was enormously popular with critics. I didn't share the enthusiasm then, and I still don't. Yes, the picture is gorgeously photographed and nicely acted. And yes, it's a pleasure to find a movie that is clean, wholesome, and nostalgic for a time and place when innocence seemed the rule rather then the exception. Unfortunately, though, the film has barely a hint of insight or depth. It is all pretty views and a miable people, inoffensive but rarely enlightening. Why, then, has "My Father's Glory" been greeted with such an outpouring of affection? I think it's because mature American moviegoers have become almost desperate for entertainment that values decency over sensationalism - and are prone to go overboard with enthusiasm when an example of this appears. I share the longing for more decency in movies, and I hope Robert's success leads Hollywood to some constructive new thoughts on this matter. But the fact that a movie is clean and pleasant doesn't mean it's intelligent or enriching; and reviewers would have given a more balanced assessment of "My Father's Glory" if they had kept this in mind. TO my surprise and delight, however, the second half of the story ("My Mother's Castle") turns out to be far more worthwhile than its predecessor, maintaining its wholesome quality but admitting that there is a dark side to life; it plays minor as well as major chords, providing many more resonant events for Marcel and his household to encounter. The family's jittery journeys toward their country home, taking them across opulent estates they are not supposed to trespass on, are visually stunning, dramati cally gripping, and metaphorically brilliant as symbols for Marcel's gingerly passage out of childhood. The end of the film is unexpectedly poetic, too, revealing trials and sorrows that await Marcel once he has entered the adult world at last. "My Mother's Castle" fulfills the promise that "My Father's Glory" only hints at, and stands by itself as one of the summer's most refreshing movies.