I was first introduced to this concept (two words that have meaning when combined in either order) as Kimball Pairs by my friend Rod Kimball (fellow puzzle nerd and operator of Enigami Puzzles & Games). It's been fun, over the years, trying to come up with new Kimball Pairs and hearing of ones discovered by others. Pairing them together to build cognitively cohesive phrases suitable for themers was suggested by David, and hence this puzzle was born.
I've seen many forms of wordplay over the years, but Wren's "Kimball Pairs" felt fresh and fascinating! CAVEMAN MAN CAVE in particular made me laugh. Working around four 14-letter entries is every constructor's nightmare, since the forced black square at the start or end of each theme entry really constrains your options—you couldn't place a theme entry in row 3, for example, without having a not-so-pretty "finger" of black squares in rows 1, 2, and 3 (in order to avoid 2-letter words). Not surprisingly, there aren't any long downs today, but Wren did a great job of keeping things smooth and squeezing zing into shorter slots with entries like HOT N COLD, WAGYU, and PC HELP. He also wrote my favorite clue in the puzzle: [___ Hammer]. Note how either ARMAND or ARM AND is a valid answer. Happy Fourth!