First, “Calvin’s Understanding of the ‘Twofold Grace of God’ and Contemporary Ecumenical Discussion of the Gospel” by Cornelius Venema (Mid-America Journal of Theology vol. 18, 2007, pp. 67-105) introduces Calvin’s duplex gratia Dei from a historical-theological perspective.

Second, the same author has a dissertation on Calvin’s duplex gratia: Accepted and Renewed in Christ: The Twofold Grace of God and the Interpretation of Calvin’s Theology.

I think we really only scratched the surface on the WSC/WTS issue. The debate centers on the question, “From what do all the benefits of God’s saving work in Christ flow?” The answer some of the WSC profs seem to gives is, “From justification.” The answer Gaffin gives is, ‘From Union.” I am making a sweeping generalization that is probably true of some of the professors at WSC and not of others there. I am also wanting to say that Gaffin does see a priority, or centrality, in justification as distinguished from the other benefits of the ordo salutis, such as sanctification and glorification. This is a very difficult issue. Start with Murray’s Redemption Accomplished and Applied. This will at least acquaint you with some of the language and more intricate issues that are surfacing in this discussion.

The union with Christ “discussion” is a complicated one. I’m glad this episode is starting to clear it up for you, although it wasn’t as organized as it could have been.