关于ScrollView中多个ViewController 的问题

在UIScrollView中执行多个UIViewControllers，我手动初始化了UIViewController，并且将它们作为子视图添加到Scrollview，至此都一切正常。但是scrollview滚动只能显示一个UIViewController，再多一个也不行。

怎么会这样？请帮忙找一下原因

DTArticle *article = [self.articles objectAtIndex:0];
DTArticle *article2 = [self.articles objectAtIndex:1];
UIStoryboard *mainStoryBoard = [UIStoryboard storyboardWithName:@"MainStoryboard_iPhone" bundle:nil];
DTArticleViewController *controller = [mainStoryBoard instantiateViewControllerWithIdentifier:@"DTArticleViewController"];
DTArticleViewController *controller2 = [mainStoryBoard instantiateViewControllerWithIdentifier:@"DTArticleViewController"];
controller.article = article;
controller2.article = article2;    

[self.parentScrollView addSubview:controller.view];
[self.parentScrollView addSubview:controller2.view];
self.parentScrollView.contentSize = CGSizeMake(self.parentScrollView.frame.size.width
                                               * 2, self.parentScrollView.frame.size.height);
self.parentScrollView.showsHorizontalScrollIndicator = YES;
[self.parentScrollView setPagingEnabled:YES];
[self.view addSubview:self.parentScrollView];