We support having separate designs for phones and tablets. To do so you need to create your project with two top-level collections (it's a setting when you make your project). This enables you to build out dual sets of collections with different layouts and card templates so you can design for each class of device.
This is, unfortunately, rather cumbersome if you are using responsive HTML for your content. We're working on a different approach where you'll be able to specify two layouts for a collection: one to use when displayed on tablets, the other on phones. I don't believe that will get done before the end of the year howevere.