In theory you can share manuals but this is limited by invariant sections and the requirement to put all authors onto the title page.
Including a few lines of the Emacs manual into Jed's manual (assuming that is GFDLed) would require to also include the GNU Manifesto and other stuff. Some people might want to state their opinon on some of Richard's thoughts too and voila another invariant sections needs to be carried on.
Sure, you can do that but the extra texts are irrelevant, dead weight and might not convey the intention of the author.
Werner, I'm really not following you. Either you can share something, or you cannot. Obviously, you can share GFDl:ed manuals, I hope we can agree on that. What you are speaking about is combining different works, this isn't the same thing as sharing. Even the GPL has some dead weight.
A few lines of material cannot be considered a legally significant change. So you are free to include those few lines and ignoring the license (basically). If you wish to incoperate a whole page, or chapter, it would be better to simply refer to it and not include it if you do not wish to have the invariant sections. The same thing applies to GPLed works which you wish to include in non-free programs, where you have to jump around abunch of hoops to be able to do that.
Cheers.