Xcode Only Uses One Source Window No Matter What


My XCode (was 2.0 now 2.1, same problem) has (spontaneously?) developed a problem where when I double click a file (or use open, or...) it won't ever open a second window, but just reuses the one open window. I have tried looking for an appropriate preference, reinstalling, deleting com.apple.xcode... all to no avail. Help! (I'm running 10.4.2)
Have you tried changing the "Layout" options in XCode's preferences under "General"?

Also, there is an option to enable or disable opening of counterparts in the same editor. This may help as well.