This post is from the Activity iPhone Duo UIKit Q&A.
How should existing UlNavigationController-based applications adapt their navigation hierarchy for iPhone Duo without rebuilding the navigation architecture?
Adapting UINavigationController-Based Apps to Duo Without Major Architecture Changes
UINavigationController works fine on iPhone Duo. There's no requirement that you move away from that architecture.
You may want to consider alternative layouts for the views within your navigation view controllers to take advantage of the regular horizontal size class on the inner display. UIArrangementViewController is a great choice inside a UINavigationController.