How to create a two-column drop down menu in main Navigation

I have a list of 16 items in a drop down menu on my main Navigation for this site. If you hover over “Member Organizations” you’ll see the drop down menu is very long and you have to scroll down to see the rest of the options. I want to avoid this issue by making the submenu drop down into 2, or even 3, columns. If this can be done through CSS, that would be preferred, but if needed a plugin can be installed.

Thank you for any help!