When I'm using an external display with my laptop, sometimes I get two rows of workspace buttons in the status bar of my primary display. This is nice, because the top row shows workspace buttons for my external monitor, and the bottom row shows workspace buttons for my laptop screen.
But sometimes (including right now) I only get one row, corresponding to the workspace buttons for my laptop screen alone.
How can I ensure that both rows are present, since I like this behavior? (And conversely, how can I switch back to "only the row corresponding to this monitor"?)