If you want the best user (and developer) experience, write 2 different codebases and share graphical assets.

Even though you could write portable C++, the API for getting to the hardware is going to be pretty different. I have worked on a project where we developed an app with identical look and feel on both Android and iOS simultaneously. The UI was basically completely custom (designed in Photoshop by a 3rd party).

My biggest advice is to do the Android UI layout IN CODE. The iOS version was completed in about 70% of the time of the Android project mostly because of the slowness of developing the UI with XML/Eclipse and Android's immature layout API.