This issue comes up so often, I've had plenty of time to revise and perfect my response to it. Dual Licensing is permitted, in theory, but in a much more limited way then what some of our authors intend. The most that an author can do is assert that a particular revision of a particular page is dual licensed, and only if all previous authors (if any) maintain that assertion. If any author edits a page that was dual licensed and chooses to go GFDL-only on their contribution, the page is no longer dual-licensable (but previous revisions still can be, if viewed from the page history).
Imagine a book donation where the donated book is GFDL+X (where X is some other free license). The very first uploaded version of that book also can be marked as being GFDL+X, and a link can be made to the original source with an indication that the original will always be GFDL+X even if future revisions at Wikibooks are GFDL-only.
Many books with complicated arrangements like this may wish to devote an entire page for the purpose, such as "/Licensing" or whatever. A simple notice such as "The original revision of this book as uploaded on mm/dd/yyyy is released under license GFDL+X and is available from LINK".
Besides this arrangement, there isn't anyway for a book to be dual licensed and to remain dual licensed while it's on Wikibooks. We could try to implement some kind of complicated opt-out ("remove this template if you choose to make your contributions GFDL-Only"), but that's not really feasible in the long-run and will only lead to confusion ("But I didn't see the notice!").
In general, besides original versions where the author wants to make clear that an alternate source for the book is released under a dual-license scheme, we can't really support dual-licenses.
--Andrew Whitworth