I want to offer a free version and a premium version.
In this case, you do not need to create two apps in two projects: all you need is a second target for your premium version. Here is a link that explains how to create and manage multiple targets in Xcode.
The process boils down to adding a target to the project, defining a separate properties plist for it, optionally setting up a preprocessor symbol for conditional compile, and using that symbol to #ifdef
portions of your classes not needed in the free version.
Another common approach to managing free vs. premium offering is to provide a single version for free, and let users upgrade it to premium through an in-app purchase.