Kurt A. Scharping posed a couple of propositions to one member of the radical two kingdoms group (I'm hesitant to name him, lest I embarass him). The propositions were these:
1. The Scripture says things that bind the civil magistrate.
2. The Church can proclaim those things that bind the civil magistrate.
He then asked one of the proponents of a strain of R2K to indicate whether he affirms or denies these propositions. The proponent appears to have answered in the negative, which (if it were really the case) would leave that proponent outside the confessional boundaries.
Even the American Revisions to the Westminster Confession of Faith teach that the Scripture says things that bind the civil magistrate, and the inclusion of these in the public document of the standards shows that the standards believe it is proper for churches to proclaim those things that bind the civil magistrate.
My point in raising this issue is not to tar everyone who calls themselves "two kingdoms" with a broad brush. My point is that even those who call themselves "two kingdoms" eventually have to moderate their tendency to separate church and state. The Bible may not say what form of government is the best, but it does provide teachings that bind the civil magistrate.
We would hope that even the most radical R2K proponent would think that the wall of separation between church and state is something that Scripture itself mandates. As such, it does not seem that any proponent of R2k, even the most radical, could deny the two propositions that Kurt has identified.