A simple NSStoryboardSegue to replace the last view controller in a NSSplitViewController.

ReplaceSegue.swift

import Cocoa

class ReplaceSegue: NSStoryboardSegue {

	override func perform() {

		guard
			let source = sourceController as? NSViewController,
			let new = destinationController as? NSViewController,
			let split = source.parent as? NSSplitViewController,

			// Assuming that the last view controller in the split is
			// the one that should be replaced, mimicking an iOS
			// two-pane split view controller.
			let old = split.childViewControllers.last,
			let oldItem = split.splitViewItem(for: old)
		else {
			return
		}

		// Make sure we're adding a new item, it's possbile the 
		// split view only has one item – the source view controller!
		if old != source {
			split.removeSplitViewItem(oldItem)
		}

		// Create the new split item from the view controller 
		// and add it. This is all a bit legacy workaround.
		let newItem = NSSplitViewItem(viewController: new)
		split.addSplitViewItem(newItem)
	}
}