Oskar Sigvardsson wrote:
Lets make an open source software analogy: if I make my own web-browser that is based on Firefox, I can't license that software in anyway I want (whether it is to make it less free by making it closed source or making it more free by using the BSD license) because the GPL doesn't allow that kind of relicensing. Same thing with the GFDL, you can't just change the rights of something that someone else wrote unless that person specifically allowed it. You are only allowed to make derivatives of someone else's work if you follow the rules, and multi-licensing doesn't follow those rules (in my understanding, anyway).
But if you were to create a new plugin for Firefox that's a separate module of code that you wrote entirely by yourself, you could multi-license just that one chunk of code under multiple licenses.
The idea with multi-licensing Wikipedia contributions is not that one would go to an existing article, fix a few typos, and poof the article is now multi-licensed. The idea is that an author could go and write a brand new article, or add a new section to an existing article, and _that_ material would be multi-licensed. An editor who subsequently edits that material but who is not themselves multi-licensing would in effect only be editing the version with the compatible license, so subsequent revisions would in all likelihood be back under just the GFDL again. But if someone really wanted to they could go back in the article's history and dig out the "clean" multi-license material and fork their own version under the other license.
Yeah, in the vast majority of cases this is a highly tedious and probably unwieldy process. But it's not illegal and under some circumstances it just might come in handy. Multi-licensing images is probably the most useful thing since images tend to be monolithic units that don't get edited much.