The White Horse Inn blog has posted a number of helpful resources on the two kingdoms and why we must keep them distinct. Beck's Two Kingdom Confusion
If we don't, we'll end up with a Mormon leading Jews, Muslims and Evangelicals together in prayer . . . Wait, that just happened!
If we understand the two kingdoms, we'll understand why it would be perfectly OK for these diverse groups to work together on political causes, if only they kept their civil religion out of it!
Somebody ought to write a book on this!