Adding a second Y axis to an LTSpice plot
LTSpice automatically does this, simply by adding a probe point with different units (right click / add trace or single click on node of interest).  The image below, for example, probes a dc voltage sweep on the primary (left) y-axis and a current response on the secondary (right) y-axis.  This is useful if you want to plot two quantities of interest (different units) vs a single variable.



If you mean a secondary plot pane, you can click on add plot pane and probe the next quantity of interest (can add several also).