If you want to add two columns to a one column page template you have to start by opening the 'Layout' section on the left sidebar and drag and drop the 2 column element to the page.

At this point we only have a placeholder. We are going to add a Heading element

and a paragraph element to each column.

Heading

Paragraph

This is the result.

Tip: This is one of the very few cases where we can recommend to copy and paste the content to the heading and paragraph elements ... but try keep the columns as symmetrical as possible and fitting inside the one page as the columns cannot cross to the next page.