Apple - Is there a PDF viewer with a vertical split screen feature?
You can take a snapshot in Skim (File -> Take Snapshot), then using the wonderful ShiftIt move the snapshot and the main view of the pdf to the opposite sides of the screen (vertically) with just a few keystrokes. Or horizontally. Or to all four corners if you want.
A quick look suggests that this feature does not exist in any PDF reader.
One responder on this forum [ http://is.gd/i0R12 ] believes that there are no such readers.
Most of the links in this Google search [ http://is.gd/i0QQy ] are requesting such a feature be implemented in some reader, any reader.