When creating a storyboard file in Xcode, you must select if it is for iPhone or iPad. This implies one should always put iPhone and iPad UIs into separate storyboards. Is
You've got to create to separate storyboards for each kind of device. If you would delete the iPad storyboard, than your app would use the iPhone's one. You'll realize it when you'll see the 2x button at the bottom of the screen. Everything will be scaled to fit the larger screen - and the graphics would be really bad.
The only suitable workaround is to copy-paste everything from your iPhone Storyboard to iPad storyboard. Just follow the next steps:
And don't forget, that a good iPad application shouldn't be the same as the iPhone version. There are a lot of cool things which you can do with iPad!