How do I make columns for only one dropdown menu?

I have 2 dropdown menus in my header of which I only want to edit one of them using custom CSS. How do I do it? Is it even possible? Please help I have this website due in 3 days.

Yep, it's possible. I assume the 2 columns should only appear on desktop, not the mobile menu.

Add this to your custom CSS:

.header-nav-folder-title[href="/folder-slug"] + .header-nav-folder-content {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0 1em;
}

Change /folder-slug to the slug of the folder you want to have 2 columns.

Hope that helps.

Andrew