First, Objective Justification and Subjective Justification are not two different justifications, but rather two parts of the act of Justification. My brother David has put it well: Objective Justification = God justifies the sinner [through faith]. Subjective Justification = [God justifies the sinner] through faith.
Objective Justification refers to the work of God in Christ as well as the proclamation of the gospel and administration of the sacraments. Subjective Justification refers to faith, which is created by that proclamation and receives the benefits. Subjective Justification does not refer to the administration of the means of grace. While it is true that when we speak of the application of the the accomplished act of Christ we certainly speak of faith, nevertheless the application of the righteousness of Christ in the means of grace as such is objective. God, in Christ, reconciles the world to himself… entrusting the word of reconciliation (2 Cor. 5:19). It is all one motion. This is why the pastor can pronounce absolution on a sinner even though he does not know for sure –outside of the sinner’s confession — if he truly has faith.
Article three of the Formula of Concord lists the necessary parts of justification (SD III, 25): the grace of God, the merit of Christ, and faith, which receives the righteousness of Christ in the promise of the gospel. The grace of God, the merit of Christ, and the promise of the gospel are all part of Objective Justification. Faith receiving the righteousness of Christ refers to Subjective Justification.
Obviously the means of grace are involved when we discuss Subjective Justification, since it is in them that faith receives the righteousness of Christ. Similarly, the plan and work of our redemption are discussed as well. After all, they are not two different justifications. However, when we speak of Objective Justification, we are not only speaking of what God did back then, but also what he declares today in the promise of the gospel. When we speak of Subjective Justification, we are speaking specifically of faith receiving what is objectively given.
The discussion of Objective and Subjective Justification is simply a distinction within one act. God quenches our thirst. This is one act. Nevertheless, we can distinguish between God preparing the water and pouring it into our mouths on the one hand, and us receiving it in our mouths on the other. It doesn’t change the fact that it is one act. The fact that a sinner can know that he is justified through faith presupposes that the righteousness of Christ is accomplished for all sinners and offered to all sinners.