In Answer to GOLLUM's query....

[j d worthington]

The question asked is:

...why did HPL use the pseudonym he did for the 2 co-authored works that appear?

Well, there are two phases to the answer to this, actually. First, there is the fact that Lovecraft, especially in his earlier years with amateur journalism, frequently used pseudonyms, often with a particular purpose in mind, but sometimes there doesn't seem to be any reason for choosing a particular one. Among his pseudonyms are: Lewis Theobald, Jun., Ames Dorrance Rowley, Ward Phillips, Isaac Bickerstaffe, Jr., Archibald Maynwaring, Richard Raleigh, El Imparcial, and quite a few others.

Second: As for why on these particular stories... that involves a bit of speculation, though with "The Green Meadow" he reveals that he did used the collaborative pseudonym because of "the Jordanian dream-skeleton" upon which parts of it are based.

As for the two pseudonyms used here: Lewis Theobald was the 18th-century Shakespearean scholar who became the object of Alexander Pope's ire and therefore the subject of his first version of The Dunciad; this (unjustly) caused Theobald, who was actually a very good scholar -- far more so than Pope on this subject at least -- to be seen as a pedantic old dunce. Lovecraft, being so fond of the Georgian era and knowing this, used the fact that he himself tended to write so much verse in the heroic couplet form, as well as using the more formal diction and construction of that period, plus the fact that he was frequently chidden for this even by friends, to publish many of the most antiquated, stiff, and Anglophilic verses under the Theobald pseudonym.

At one point, a biography for Lewis Theobald, Jun. was published in the United Amateur. I don't believe this was by Lovecraft himself, as it isn't included in his DPC columns (or any others) in his Collected Essays that I'm aware of. I think it most likely to have been written by either Maurice Winter Moe or Alfred Galpin, either of whom was quite capable of such a tongue-in-cheek bit of scholarly prankishness:

Amateurs who have seen the name Lewis Theobald, Jr., in the Providence Amateur and, more often, Howard Lovecraft's Conservative, have probably wondered as to the identity of this new and unheralded bard. Mr. Theobald, who has not yet been induced to join either association [meaning the United Amateur Press Association or the United Amateur Press Association], is a scholar and poet of considerable attainments, and was born a little over thirty-five years ago in Blauvelt, France, where his parents were living at the time. When Mr. Theobald was ten years of age the family returned to England, and he was sent to the Charterhouse, one of England's great public schools, where both Addison and Thackeray, at different periods, were students. Mr. Theobald claims an even closer connection with English letters than this, however, and says he is not ashamed to admit that the Lewis Theobald mentioned in Pope's "Dunciad" was his great-grandmother's uncle. Several years ago, Mr. Theobald came to America and acted until recently as 3d assistant librarian of the Providence Public Library, where Mr. Lovecraft first made his acquaintance.

(A Winter Wish [ed. Tom Collins], p. 156]​

Lovecraft frequently signed his letters with the Theobald pseudonym, often making it "Grandpa Theobald". This had its own amusing sidelight in that, when HPL sent the ms. of At the Mountains of Madness to Robert E. Howard (following its rejection by Farnsworth Wright of Weird Tales), he included the following "Schedule of Circulation":

Augustus Derletus to Donaldus Vandreius
Melmoth the Wanderer to Klarkash-ton
Klarkash-ton to B'na-Dwi-yhah
Bernardud Diverius to Grandpa Theobold.

(Robert E. Howard: Selected Letters 1931-1936, p. 14)​

As you can see, Howard misread "Theobald" for "Theobold", for one thing; for another, puzzled as to the identity of this person, he noted: "Which of course are August Derleth, Donald Wandrei, Clark Ashton Smith, and Bernard Austin Dwyer; but who is Grandpa Theobold?", proceeding (after a quotation from a verse about Villon) with the following:

Cities brooding beneath the seas
Yield the chalcedon and gold;
Ruthless hands the treasures seize,
Rending the Ages' mysteries,
But who is Grandpa Theobold?

Secret of the eternal Sphinx
is a story worn and old,
Like a tale too often told;
All the ancient unknown shrinks --
But who is Grandpa Theobold?

Fingers turn the hidden Keys,
Looting wealth from lair and hold;
Cast what shapes in what dim mold?
Question now the Eternities.
But who is Grandpa Theobold?

Prince, before you snare the stars,
Speak, before the sun grows cold
Scowling through the morning bars,
Who is Grandpa Theobold?

(ibid., pp. 14-15)​

However, the odd history of this particular joining of pseudonyms and the people who used them doesn't end here. While Elizabeth Neville Berkeley was generally used by Winifred Virginia Jackson (Jordan, when HPL first knew her, before her divorce from Horace Jordan in 1919), but there were two poems published under that pseudonym that were actually by HPL: "The Unknown" (The Ancient Track, p. 18) and "The Peace Advocate" (a parody of the anti-war sentiment during WWI; see AT pp. 406-09). As he explains in a letter to the Gallomo -- a correspondence group made up, at least originally, of Alfred Galpin, HPL, and Maurice Winter Moe -- dated 12 Sept. 1921:

It is true that I once used the pseudonym of "Elizabeth Berkeley" in conjunction with its more rightful owner W. V. J. -- in 1916 the name covered certain verses by both authors, in an effort to mystify the public [i.e., the amateur journalistic public] by having widely dissimilar work from the same nominal hand.

(Letters to Alfred Galpin, p. 108)​

In the case of both "The Green Meadow" and "The Crawling Chaos", each had its origin in dreams of HPL's, which -- according to what Ms. Jackson said -- were extremely similar to ones she herself had. Though HPL later became skeptical about these dreams, he didn't doubt her sincerity. At any rate, here, in part, is his account of the origin of "The Green Meadow":​

A singular dream had led me to start a nameless story about a terrible forest, a sinister beach, and a blue, ominous sea. After writing one paragraph, I was stalled, but happened to send it to Mrs. Jordan. Fancy my surprise when the poetess replied that she had had a precisely similar dream, which, however, went further. In her dream a piece of the shore had broken off, carrying her out into the sea. A green meadow had loomed up on the left hand side, and horrible entities seemed to be hiding among the trees of the awful forest behind her. The piece of earth on which she was drifting was slowly crumbling away, yet this form of death seemed preferable to that which the forest things would have inflicted. And then she heard the sound of a distant waterfall and noted a kind of singing in the green meadow -- at which she awaked. It must have been quite some dream, for she drew a map of it and suggested that I write a story around it. After a little consideration, I decided that this dream made my own proposed story a back number, so I abandoned my plan and used my original opening paragraph in the new story.[...] I decided to add piquancy to the tale by having it descend from the sky in an aerolite -- as Galba knows, for I sent the thing to him. I accordingly prepared an introduction in very prosaic newspaper style, adding the tale itself in a hectic Poe-like vein -- having it supposed to be the narrative of an ancient Greek philosopher who had escaped from the earth and landed on some other planet -- but who found reason to regret his rashness. As it turned out, it practically my own work all through, but on account of the Jordanian dream-skeleton I felt obliged to concede collaboration, so labelled it "by Elizabeth Neville Berkeley and Lewis Theobald, Jun."

(ibid., pp. 82-83)​

With "The Crawling Chaos", on the other hand, he gives more credit to Ms. Jackson, saying that he "didn't do more than half or three quarters of it", and that it is (again) largely built around "W. V. J.'s richly and exotically ornate dream nucleus" (LAG, pp. 108-109).​

Incidentally, yes, the title is at least somewhat linked to the prose-poem (and the entity) "Nyarlathotep", as he "took the title C.C. from my Nyarlathotep sketch (now repudiated) because I liked the sound of it" (letter to Robert H. Barlow, [1 December 1934]; see O, Fortunate Floridian, p. 191).​

So it would seem that he used the joint pseudonyms because of the fact Ms. Jordan/Jackson provided at least a fair amount of the impetus for the creation of the two tales; and, as she was often going under her pseudonym in the amateur press, and so was HPL....

At any rate, Mr. G., that's what I have to offer for now. You may, at this point, be regretting having asked the question; but I at least hope I've given the germ of a satisfactory answer... and indicated HPL's tendency toward a sometimes Puckish sense of humor....

 

[GOLLUM]

At any rate, Mr. G., that's what I have to offer for now. You may, at this point, be regretting having asked the question; but I at least hope I've given the germ of a satisfactory answer... and indicated HPL's tendency toward a sometimes Puckish sense of humor....

No not at all. It's a wonderfully researched and quite excellent answer. I knew you woul have the details at your literary fingertips, so I thank you for the insights provided here....