Like Excel does when you use 2 columns for the labels.
Each item on the category axis falls in a group. The groups are displayed below the axis items. Each group name is displayed only once (centered below all its items). Groups are divided by longer tickmarks.
Each group can contain a different number of items.
* display structured categories like product and product group or country and region
* display two or more groups side by side, like gross sales and net sales, or this year and last year.
Black Label kindly shared their grouped X-axis plugin: https://github.com/blacklabel/grouped_categories