Using an NSArrayController in Multiple Storyboard Scenes

The key to making this work is to have a NSArrayController instance in each of your descending NSViewController subclasses and binding them together through a central data source (most likely your NSDocument subclass). You can then set this data source as your NSViewController subclasses representedObject by passing it down through your descending controllers. Here is an example of a storyboard application with an NSWindowController which has a content view controller that is a NSSplitViewController with two child view controllers (A Master / Detail setup):

class Document: NSDocument {

    var dataSource: DataSource? = DataSource()

    ...
}

class DataSource: NSObject, NSCoding {

    var items: [Item] = []
    var selectionIndexes: NSIndexSet = NSIndexSet()

    ...
}

class WindowController: NSWindowController {

    override var document: AnyObject? {
        didSet {
            if let document = self.document as? Document {
                self.contentViewController?.representedObject = document
            }
        }
    }

}

class SplitViewController: NSSplitViewController {

    override var representedObject: AnyObject? {
        didSet {
            for viewController in self.childViewControllers as! [NSViewController] {
                viewController.representedObject = representedObject
            }
        }
    }
}

The trick is to bind the representedObject to each of your descending view controller's NSArrayController in the storyboard. You need to bind NOT ONLY the contentArray BUT ALSO the selectionIndexes.

The result is that the selectionIndexes on both descending NSArrayControllers are kept in sync because they are bound through the central data source (DataSource subclass in above example).

To make this all clearer I have created an example project that demonstrates this here: https://github.com/acwright/StoryboardBindingsExample