How to add two layout inside a sideBySideItem

You can't. You will have to switch to columnsLayout() 

And if your rich image is just a small square-shaped image you want to display, then you can add it in the same richTextDisplayField using a!richTextImage and a!richTextItem. Use char(10) between them to add a new line. 

Hello Arib 

It is not supported to do so in Side By side layout. You will have to use a!columnsLayout() to do so.
Look at these for more ideas on using columns layout.

Sail Design System-UX_Columns_Layout

Can you please guide me with the layout so that I can align one of the Item on the right and the other two(Img & text) being on top of each other. while keeping these two on the left.  If I use the columnsLayout I would be keeping the Image on top of the two items(rich Text component & sideBySideItem).

while the sideBySideItem on the 'right' has a lot of details which should be highlighted.

Also, this image pretty decent size, I had tried with richTextItem but it becomes too small to be visible.

{
  a!columnsLayout(
    columns: {
      a!columnLayout(
        contents: {
          a!imageField(
            label: "Image",
            labelPosition: "COLLAPSED",
            images: {
              a!documentImage(document: a!EXAMPLE_DOCUMENT_IMAGE())
            },
            size: "MEDIUM",
            isThumbnail: false,
            style: "STANDARD"
          ),
          a!richTextDisplayField(
            align: "CENTER",
            labelPosition: "COLLAPSED",
            value: { a!richTextItem(text: { "Styled Text" }) }
          )
        },
        width: "NARROW_PLUS"
      ),
      a!columnLayout(
        contents: {
          a!paragraphField(
            label: "Paragraph",
            labelPosition: "ABOVE",
            saveInto: {},
            refreshAfter: "UNFOCUS",
            height: "MEDIUM",
            validations: {}
          )
        }
      )
    }
  )
}

Use columns layouts for the course layout, and side by side layouts for the fine, single row adjustments.

- For this particular use case, you don't need sideBySideLayout, the columnLayout will do the job.

- Have the columnLayout at top level and sideBySideLayout within it if requires.