Two line reasoning is not a staple in historical reformed theology. I believe that two line is teaching a contradiction. Reformed theology is a defense of the gospel of justification by faith. We believe that as we apply justification by faith in its proper context that we are describing true faith. Our objective is to defend justification by faith in teaching that salvation is not a work of man but it is by grace alone through faith alone in Christ alone.
But the word justification is not just a term that describes faith but it is a legal declaration of innocence. In defending justification by faith we believe that before God we are free from blame. But the two line teaching is that we can still be blamed because we are also under the tension that sanctification brings.
Let me give you a few reasons why we are already blameless. I do not believe that God in speaking about His relationship with Israel in the ot was pointing to His elect when He addressed His people as stubborn, disobedient and dull of hearing. This is where I draw the line when I defend Justification by faith. I can see where people who believe in two line can draw this conclusion by how you have God addressing His people in the law in Deuteronomy and demanding compliance to the law as the basis of covenant faithfulness. You have the specific laws stated and the requirements for compliance. But I believe that we are obligated to see the Deuteronomy teaching in the context of how they applied it in their worship. So I believe that the law was taught in the words of the Psalms as the culture that we see describing the way they thought in their daily lives.
In memorizing the book of Psalms and becoming proficient in the use of the phrases you will find the Spirits teaching in the rest of the ot within the applied order of words that are in the Psalms. It is very obvious to a person who has this kind of head knowledge. I do not believe it is accurate to teach an OT passage without understanding the culture of words that are in the Psalms. I believe as we are unfamiliar with this culture that we water down the way that they approached God in a personal sense.  I will explain this further.