Sure. You could have a scaleable/responsive UI, separate layouts for each platform i.e. xibs or completely separate interfaces. But you would still need that common SDK.
It looks that everyone is working to that goal. But no one is sure yet.
I'm not sure it would be a disaster. If it's implemented in the same way as universal iOS apps are currently done, with separate NIBs for each platform, and if the main UI components are merged (i.e. UIButton and NSButton) then I can imagine UI development on multi-platform apps being quite easy to do, and easy to to well also.
MS/Win8 has shown how frustrated people get when you try to force a common UI - It's been generally regarded as a clusterfuck.
Instead, decouple your display code from your app logic, and build related, but distinct displays for each application.