I tried several experiments; the problem involves multiple parts:

1. Is there a dividing line between the columns?
the Document AI is not capable of detecting the dividing line between the two columns, so the problem remains as mentioned.
However, with a simple code, the image can be split into two separate images, and each can be processed individually—or a new image can be generated where the first column is placed at the top and the second column at the bottom (what is the maximum allowed image height in Google's program?).

2. Placing each text block in a box
Document AI also fails to recognize each column separately.

3. Coloring a text block
Document AI also fails to recognize the column.

4. Increasing the spacing between the two columns
This is generally successful, but it makes mistakes in some lines (this error occurs regardless of the presence of a column; sometimes a word randomly moves from its position to another line).

There are many things that would be good to explore solutions for. For example... how do we determine if a text block is a footnote?