Not sure if this could be done in iPhoto 5 or not, but I was sorting my library in iPhoto 6 and I had two images selected. I clicked Edit and both images opened in the Edit mode, side-by-side. It's exactly the same behaviour as you get with the Compare feature in fullscreen mode, but without switching to fullscreen. You can also add and remove photos to your selection in the same way you would in fullscreen mode (Command-clicking in the slide bar at the top of the screen).
[robg adds: I also accidentally discovered this behavior when I Command-clicked one photo while editing another. I assumed then that this was a documented feature of the program, so I let it pass. However, in reading the manual today, it doesn't appear that it's documented at all. The manual goes so far as to state (my emphasis added) "If your library, an album, or an individual photo is selected in organize view, you can click the Edit button to switch to edit view." It does talk about Compare mode, but nothing about editing multiple images at once.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060121153530102