The textual criticism of the LXX isn't one of my strongest areas, but I'll take a stab at it.
Together, the LXX1, LXX2, LXX1 Apparatus, and LXX2 Apparatus modules comprise the complete text and apparatus of Septuaginta, the edition of the LXX originally edited by Alfred Rahlfs and revised by Robert Hanhart in 2006. This is the standard manual edition used by scholars. Most of the text and apparatus of the edition are found in the LXX1 and LXX1 Apparatus modules. Although the apparatus of Septuaginta is much less extensive than that of the Nestle-Aland editions of the New Testament, the format is essentially the same: readings that differ from those given in the text are listed, along with the manuscript witnesses that contain them.
However, Rahlfs determined that in some books, the manuscript witnesses were so different from one another that they represented different recensions, or editions, of those books rather than variants of a single textual tradition. For example, in the book of Judges, Rahlfs found that one recension was represented by Codex Vaticanus (and some other manuscripts), and another recension was represented by Codex Alexandrinus (and some other manuscripts). Rather than choose between them, he printed the two recensions alongside one another. Each was accompanied by an apparatus that showed the variants between the manuscripts of that recension. The text and apparatus of these additional recensions is included in Accordance as LXX2 and LXX2 Apparatus.
Once it is complete, the Gottingen Septuagint should supercede the Septuaginta, as its apparatus is much more extensive and thorough. It has been released gradually, book by book; some of these volumes are available in Accordance.
Does that answer your question? If not, let me know and we'll try again!