You could use an image, although it may be more likely that the email layout is done using an HTML table and you can use borders within the table, or if you would prefer a graphic, you can put a cell in between your 2 content cells and have an image in the middle.
I've found the best solution here is to create a background image that is 600 pixels wide, transparent with a single pixel of color in the center (for a 1px divider line) that you can apply as a background "pattern" to any section, and click the option to have it stretch or repeat.
You could use an image, although it may be more likely that the email layout is done using an HTML table and you can use borders within the table, or if you would prefer a graphic, you can put a cell in between your 2 content cells and have an image in the middle.
You would need touse HTML and create a custom module. It's definitely more complicated but works. W3 Schools is a great resource for HTML and CSS. Here's whay they have on tables: https://www.w3schools.com/html/html_tables.asp