Target latest iPad and iPhone with single application?

Is it possible to create a iPad application once and have it auto scale down to an iPhone without needing to create a separate build targeting a specific device? If so, I assume I should always target the largest device at all times?

well i did export to ipad air 2 and iphone 5 together with same files. All you need to do is just only add more testing device on your App IDs, Devices from the website.
I got no idea what you mean scale down.
Does it mean auto adjust the screen size and position ? if so , i guess you need to work hard a little bit more. But as far as i export it does great.