Dual licensing with two copyleft licenses

[Evan Prodromou]

I've got a quick question for folks familiar with copyleft licenses.

Consider a work "dual-licensed" by the copyright holder, Alice, under
two copyleft licenses: the GNU General Public License and the Open
Software License (OSL). Bob makes a derivative work. My contention is
that he can release this derivate work under either the GNU General
Public License, or the Open Software License, but not both.

My reasoning goes as follows: "dual licensing", as we commonly use the
term, is alternative, not cumulative. Noone can comply with the
requirements of both the GPL and the OSL, since they conflict at some
points (most notably in the licensing of derivative works). They both
claim some exclusivity: for example, in the GPL, "You may not copy,
modify, sublicense, or distribute the Program except as expressly
provided under this License." The OSL is a little fuzzier on the matter.

My understanding, then, is that Bob must choose one or the other license
offered by Alice. Since both of these licenses require derivative works
to be licensed identically, Bob's derivative work must have the same
license as the one he chose to use.

Am I way off? Lest this seem like a completely academic discussion, it
actually comes up from dual-licensing works under the GNU Free
Documentation License (FDL) and the Creative Commons
Attribution-ShareAlike.

http://wikitravel.org/en/article/Wikitravel:Dual_licensing

Thanks for your time,

~ESP

P.S. I believe there may be other ways that Bob and Alice can make
arrangements so that the derivative work Bob made can be dual-licensed,
too. Assignment of copyright by Bob to Alice is one way, for example.

-- 
Evan Prodromou                  .O.
http://bad.dynu.ca/~evan/       ..O
evan at bad.dynu.ca                OOO