How can I get these two columns to sit under each other rather than alongside each other? (PS. I do not want them to be the full height of the screen)

How can I get these two columns to sit under each other rather than alongside each other? (PS. I do not want them to be the full height of the screen)

I want my columns stacked one under another like this ‘cheat’ method but without having to use dummy spacer columns

I want my columns stacked one under another like this ‘cheat’ method but without having to use dummy spacer columns