My wife and I met with several friends last evening to discuss the doctrine of the two kingdoms and some of its implications for congregations and individual Christians.
In preparation for the discussion each of us read A Biblical Case for Natural Law by Daivd VanDrunen. In addition to VanDrunen's monograph we used a talk Mark Dever delivered at the 2009 Sovereign Grace Pastor's Conference (Kevin DeYoung has posted Dever's notes here) to help us work through some of the practical implications of two kingdoms doctrine. Though Dever doesn't use the phrase "two kingdoms" much of what he discusses seems to flesh out the concept.
Both resources are thought provoking and worth consulting if you're interested in this subject.