There is no lack of to-do apps designed at least somewhat around Getting Things Done - just see the answers to this question, for example. I'm a user of Things on iOS and have heard a lot about OmniFocus, but I've never gotten a good recommendation for an app that can manage to-dos for more than one person.

My wife and I are looking to adopt a GTD-inspired workflow for a variety of personal projects, some shared, some individual, and it would be ideal to manage these projects side-by-side.

Short of us each running our own instance of a given app on our separate devices, and including iOS / OS X synching among my requirements, do any of the flagship GTD to-do apps offer the concept of projects and to-dos that belong to a specific person of several?