I think before Wikibooks goes jumping head first into outright accepting dual licensed books, Wikibooks first needs to as a project find solutions to its current problems:
1. People are left hanging when it comes to what they need to do in order to be in compliance with the GFDL. This requires people to be experts in GFDL compliance.
2. People are left to figure out what licenses images and other media use, or if public domain or fair use is being used, and what that means for there reuse, assuming people are even aware that other considerations are necessary when reusing those parts of a book.
3. People are left on their own when it comes to any legal ramifications that might steam from getting any of this wrong, reducing the likelihood that Wikibooks content will be reused, redistributed, and modified outside of Wikibooks, except by the most savvy of users.
4. People seem to unable to agree as to whether or not these are in fact issues or people are making mountains out of anthills. People who do agree that a problem exists cannot seem to agree on a solution.
I think Wikibooks needs to solve these problems before even considering trying to tackle dual licensed books and how to make it work, instead of ignoring the current problems.
There are probably additional problems I've failed to mention, which either aren't coming to mind or I'm not even aware of.
-- darklama on en.wikibooks