In addition to Bob's point about the beta, DPS 2015 doesn't support iPad only apps. All content will scale correctly and display on all devices, so there's no real penalty to having your app available for iPhones as well. This is very different from the previous version of DPS, where you had to make renditions for each iPad screen aspect ratio you wanted to support, which is a pretty big endeavor.
You can do this today. When you create your project make sure to select that you want two top-level collections instead of one. That will give you the ability to create phone collections and tablet collections (put the same articles in each one), and then apply a different layout to the phone collections vs. tablet collections.
This is a bit clunky, I'll admit, if your content is shared. However it will work and will let you share content across the two platforms.