In the Page editor, the layout shuffle options will appear over your page. Using these icons, you can apply a new random layout to each page size independently, or across the spread.
The options are to shuffle either the left or right sides of the page, or to shuffle the spread respecting the gutter line or across the gutter.
The auto-layout icons over your canvas are responsive to your current layout structure. If your layout respects the page gutter (middle of the pages), all options are available.
If your layout contains an image straddling the gutter line, then only two options will be available - shuffle to respect the gutter, or over the gutter. Because an image exists on both the left and right side, you aren't able to shuffle only one side at a time.
To try out new layouts on each side separately, first reshuffle the layout respecting the gutter. All options will be made available again.
Similarly, when an image straddles the gutter, the Layout options will automatically display Spread options exclusively:
Again, reshuffle the layout create layouts on separate sides the Left and Right side layouts will then be available..