I understand what you mean by "simul justus et peccator," and realize that you are not using it as directly analogous, but as a demonstration that two opposite things can be true; however, I'm not sure that this example is entirely convincing. "Simul justus et peccator" refers to quickened, regenerated believers -- i.e., those with faith -- who haveboth the old man and the new man contending within them (Eph. 4:22-24; Rom. 7:13-25). I have no problem believing two opposite things can be true, if the Scriptures directly teach them -- as in this case (and in this case, it's no great crisis of logic to accept it, since it is rather reasonably explained in the Scriptures), or, say, in the case of the Doctrine of Predestination where the Scriptures clearly and directly teach both that it is the will of God that all men be saved AND it is the will of God that sinners suffer eternal damnation. The simple fact is, the Scriptures nowhere teach that mankind is Justified before God apart from Faith. This assertion is an invention, derived from fragments of Scripture that are stripped of their context, pasted together, and then set against what the Scriptures DO teach in direct positive terms. In Universal Justification, the universality of man's lost condition and certain condemnation is propped up against the universality of man's righteous standing before God, as if both are Scripture teaching, when in reality only the former is directly taught in Scripture while the latter is a rationalistic invention. They are not, equivalently, the direct teaching of Scripture.
As for 2 Cor. 5:19... before we go down that road, I would like to point you to four posts on this specific reference that we published in 2013, in case you haven't read them yet. I happen to agree with Chemnitz, Melanchthon, Luther and the Tübingen theologians on their interpretation of this verse. Do you?
Philip Melanchthon on 2 Corinthians 5:19
Luther's translation of 2 Corinthians 5:19
Reconciling the world—but not without means
The Lutheran understanding of 2 Corinthians 5:19