You're conflating features of the Stack Exchange engine, which copyright law doesn't care about, with ownership and licensing.
All user content (i.e. the posts and comments, not the user interface bits) on Stack Exchange is owned by their authors (there is no transfer of copyright — that's not even a thing in some jurisdictions; it is in the US but SE doesn't require it). A majority of posts are owned by the single author who wrote them. Posts with multiple authors (the original poster and one or more editors) may have shared ownership if the contribution of others is significant.
All user content comes under a CC BY-SA with attribution requirements. There is no dual licensing. The original author may use the same content elsewhere under a different license; in this case the path is from the original author directly to this elsewhere. That is not a dual licensing of the Stack Exchange post, that is a separate work by the same author which happens to have the same content.
Whether a post is community wiki does not affect this. Community wiki changes some Stack Exchange rules (lower reputation threshold for making edits without supervision, and no reputation granted for votes), and is a signal from the original poster that others are permitted and even encouraged to make significant contributions. It affects the user interface a bit; for example the name at the bottom of the post is the user who contributed the most content by some measure of length, whereas it's always the original author for posts not marked CW. The “Community” user is not involved. All of which does not matter from a legal perspective.
On the topic of posts with multiple authors — which, again, is not a matter of them being community wiki or not: the only impact of CW is that it makes it more likely for a post to have multiple authors — let me point you to the Meta Stack Exchange thread Does the migration of edited posts violate attribution requirements? and in particular my answer there. The law (in all jurisdictions, I think) sets a threshold for when contributions to a work are considered significant enough for someone to be considered a co-author. The exact threshold is determined by jurisprudence and is not always consensual. For example, copyeditors on books are traditionally not given any recognition, whereas movies tend to credit many people whose contribution is purely material, not creative.
Regarding the US in particular, the relevant law is Title 17 of the U.S. Code, and in particular chapter 1. Neither it, nor the CC BY-SA license, define who is considered an author. The article “Defining “author” for purposes of copyright by Russ VerSteeg presents the main views regarding how a contributor to a work gains authorship status. Emphasis is mine.
- “As a general rule, the author is the party who actually creates the work, that is, the person who translates an idea into a fixed, tangible expression entitled to copyright protection.” (Justice Thurgood Marshall, in Community for Creative Non-Violence v. Reid, 490 U.S. 730, 737 (1989).)
- “A collaborative contribution will not produce a joint work, and a contributor will not obtain a co-ownership interest, unless the contribution represents original expression that could stand on its own as the subject matter of copyright.” (Paul Goldstein, Copyright: Principles, Law, and Practice § 4.2.1.2, at 379 (1989).)
This is the majority view. VerSteeg's article goes on to present minority views (upheld by some US courts) which have a broader definition of authorship, requiring a lesser contribution.
The application to Stack Exchange is to the attribution requirements. The license is the same regardless of who the authors of a post are, but all authors of a work must be attributed if the work is reused. The onus is on the person reusing a Stack Exchange post to ensure that they abide by the licensing requirements and in particular credit all contributors who have authorship status. For a non-community-wiki post, that's usually only the original author, but legally speaking it's still the responsibility of the person reusing the work that's engaged.
