There's no such thing as "renditions" anymore. Instead, use the two top-level collections to structure your content however you want for phones and tablets. Put whatever you want to have show up on phones under the phones top-level collection, and whatever you want to have show up on tablets in the tablets top-level collection.
You can mix and match stuff across both top-level collections. For example if you have a collection of articles designed for phones, but you don't mind having it show up on tablets too, just put that same collection somewhere under the tablet top-level collection and it'll show up on tablets. If you want to have completely separate versions of articles across the two device types, make two completely different sets of collections to contain those articles and then put them under the tablet TLC and phone TLC respectively.