Xcode 3 had the nifty widget
that split an editing screen in two. I don't see that in Xcode 4. Has Apple decided that I don't need that anymore? ;-)
Middle button under the "Editors" in the top right will split beside each other. I don't likd that as much as the stacking one in xcode 3 though...
EDIT: You probably have to navigate to the same file to get it "split". Again, not as easy as xcode 3...