Is there an iOS / OS X to-do app that can manage multiple people's To-dos?
There are several, even though some of them may not be as native to OS X and iOS as Things or Omnifocus.

Asana, Trello and Flow are good examples. They offer nice web apps to be used through a web browser, and native iOS apps. Both offer the GTDesque approach, and allow assigning tasks to individuals.

Another alternative, which does offer an OS X app, is Producteev.