BDAG reveals that ἐπιφάνεια is used as a technical term for "a visible manifestation of a hidden divinity, either in the form of a personal appearance, or by some deed of power by which its presence is made known." But if you consult entry 1b (or its equivalent in the older BAGD), you will find out that Bauer, Danker, Arndt, and Gingrich associates ἐπιφάνεια vis-à-vis the "manifestation" of Christ with a time of 'appearing in judgment.' You also discover the statement about 2 Thessalonians 2:8 under this lexical entry.
While this same reference work does state that παρουσία may denote Christ's Messianic Advent in glory when he comes to judge the world at the end of the age, it also indicates that there is some type of distinction between παρουσία and ἐπιφάνεια as revealed in 2 Thess. 2:8.
The ἐπιφάνεια seems to take place amid the παρουσία and it is actually connected with the divine meting out of judgment--παρουσία, however, does not seem confined to the judgment executed by Christ. It is an extended period of time in which Christ rewards his servants, but wages righteous warfare against his enemies.