[JimmXinu]

Here's what I've found:

        # Internally, the second split view is referred to as the 'pin_view'.  
        # state can be gotten and appears the same as library_view.
        state = self.gui.library_view.get_state()
        pin_state = self.gui.library_view.pin_view.get_state()

        # it looks like apply_state(state) will also work the same.
        state = {'hidden_columns': hidden_cols,
                 'column_positions': positions,
                 # no sort_history in pin_view
                 'column_sizes': resize_cols}
        self.gui.library_view.pin_view.apply_state(state)

        # showing/hiding the pin_view is easy:
        # self.gui.library_view.pin_view.isVisible()
        self.gui.library_view.set_pin_view_visibility(bool)

As far as I can tell, calibre saves *whether* the view is split(), but doesn't save *where*. IE, on restart, the split bar is centered regardless of where it was last dragged. Is that intended, I wonder?

So saving and applying column data for both sides in View Manager looks doable, as does showing/not showing the splitter. Placement of the splitter, I haven't figured out.

Integrating split on/off and a second 'Columns in View' list in the plugin config dialog would be the hardest part.