You can learn more about markdown syntax here.
This option might not exit directly.You might have to use tables to accomplish this or draw a line then shift text on column or increase the spacing on that column somehow.
If you are already in Google docs, please do the following:
On the top panel, click Format.
Then click on Columns.
Then click on More options.
A dialog box opens up. Now select your Number of columns, Spacing then make sure to check Line between columns.
* Finally click Apply.
How about if I wanted to insert the line in google docs but did not want it to be divided into two equal parts?