In favor of a single author:
1. Smith met one man as the author.
2. Drake's statements to Sprong that Andrews was "the" author.
3. Statement by Del Adelphia that he met Erdnase, and it was only one guy.
4. If Erdnase had been more than one guy, it would have been less likely that his identity would still be unknown at this date ("one can keep a secret, two can't").
5. The use of "we" is obviously the editorial we
(although some grammarians might say it is improperly used). I don't see anywhere where "we" is used that the context requires that it mean more than one person.
6. And despite its use, the author clearly refers to himself in the singular as well, in one of the most well-known quotes from the book: "if it sells it will accomplish the primary motive of the author, as he needs the money." See also "the writer [singular] uses no sophistry" and "The writer has expended much time and care . . .".
7. Neither the copyright application nor the attribution of the author on the title page refer to more than one author.
8. While reference is made to partners, allies, collusion, etc., the book is written primarily for the lone player. The methods taught are for the single advantage player, rather than for partners (for example, the "spread" isn't included in the book). Erdnase preferred to work alone at the table -- why assume he wanted partners in writing?
9. The author claims originality with respect to several sleights, a statement which makes much more sense if the author were one person rather than several. The sleights in question aren't performed by a group, but by a single player or performer.
To me, the evidence leans to a single-author theory.