At this point we are treating phone and tablet apps as seperate apps (basically they are individual apps that have different aspect ratio screens), so I am afraid you will need to create a seperate app for the tablet version (though of course you can copy experessions etc across to the tablet version)
However, either version will play on either device (i.e. you can open the tablet version on a phone and vice versa). One thing you could try is saving the phone app under a different name, then go into File->App Settings and change the Orientation to landscape, set Lock Orientation to "On" and set Lock Aspect Ratio to "Off".
See how the app looks on a tablet and see whether tweaking your controls can get you to something acceptable to your purpose. I wouldn't recommend that for major work, but it may be a first and dirty solution to try before starting the seperate tablet version.