You are on the right path.
Within tableViewSelectionDidChange() you need to instantiate a new viewController from your storyboard using NSStoryboards instantiateController(withIdentifier: String) method. Then you can set this as your splitViews second view controller.
Then you need to create a splitViewItem. You can use the init method which takes a viewController for this (NSSplitViewItem(viewController:)).
Finally you have two possibilities to add the new viewController.
Either you use the add/removeSplitViewItem methods or you set the splitViewItems array directly.
My words in code:
guard let splitViewController = self.parent as? NSSplitViewController,
let viewController = self.storyboard?.instantiateController(withIdentifier: "yourIdentifier") as? NSViewController
else { return }
let item = NSSplitViewItem(viewController: viewController)
// Method one
splitViewController.removeSplitViewItem(splitViewController.splitViewItems[1])
splitViewController.addSplitViewItem(item)
// OR method two
var items = splitViewController.splitViewItems
items[1] = item
splitViewController.splitViewItems = items