Can anyone give me a concise summary of Martin Luther's "Two Kingdoms" theology, and also provide some references back to his original works on the subject?
Without having understood the doctrine of the two kingdoms, it's not possible to understand Luther's theology in its full extent.
It's a misunderstanding to think that by the justification by faith alone all laws have lost their importance. As short as possible: a Christian lives under two completly different kingdoms, where the one is "God's kingdom under Christ"1 – here he lives by faith in the righteousness coram Deo2 – and the other is the world's kingdom – here he is man or woman, young or old, lord, servant, peasant, citizen etc.3 and has to do good works.
Accordingly, the human being is both: Christian person and secular person.
You see: the accusation, that Luther's justification by faith alone shrinks the room for the works that have to be done by us, is a misunderstanding. Especially Luther's theology shows people, how overfull of opportunities for good works the world and the way, which they follow in the world by faith, is.
1: "Gottes Reich unter Christo"
2: before God
3: WA 32, 440, 2
4: "Christperson" and "Weltperson"