It seems pretty standard for commentaries to be written on the combination of 2 Peter and Jude. Obviously they both share the subject matter of false teachers, but I assume there are reasons beyond this for including them together, likely involving source criticism. Do scholars think one borrowed from the other? From a common Q-like source? Why are these books usually written about together?
It is often believed that the author of the second book references the first book. This is based on both using otherwise rare words (such as "multiply" in Jude 2 being used in 1 Peter 1:2 and 2 Peter 1:2) and themes ("our common salvation" in Jude 3 and "a faith of the same kind as ours" in 2 Peter 1:2). Whether Jude quotes 2 Peter or 2 Peter uses content from Jude (Bauckham also lists out the possibilities of both drawing from a common source and having the same author. He holds neither position), is debated (see the Introduction to 2 Peter in Expositor's Bible Commentary, Ed. F.E.Gaebelein, Zondervan 1976-1992). Conservative scholars usually date Jude between 60 and 90.
Jude and 2 Peter are very similar (see especially "Relation to Second Peter), especially Jude and 2 Peter 2. Some argue that Jude was the source** based on style, brevity (the belief that the shorter work came first), and believing that 2 Peter is psuedopigraphical. However, Wallace answers those arguments and makes several arguments for Petrine Priority.
** e.g. Terrance Callan, "Use of the Letter of Jude by the Second Letter of Peter," in Biblica 85 (2004), pp. 42-64.
Is it at all possible that these two books were written completely independent of one another by the individual prompting of the Holy Spirit? From a Christian perspective, the Holy Spirit is indeed the true Author of God's word and merely works through men by divine inspiration (as affirmed by Peter in 2 Peter 1:20-21). Given this, is it not quite feasible that this was the case with these two books? I'm a writer, and it is quite feasible to have two articles on the same subject matter published by different publishers, yet have contents so similar because both writings flow from the same wellspring of thoughts and ideas.
It's great to establish the scholastic/historical authorship of these books, but I think it's also important to remember the ultimate Source behind these human vessels. ALL scripture is inspired by God, therefore He's the real Author and may choose to underscore an important theme using two or more different individuals in different times and places.