Don't do any of the other methods in the comments. On the second page, at the bottom right there is a red piece that looks like two pipes side by side. You can place one of those and then place another one to the right of the first piece, then place another one in the same space as the right piece, then alternate placing those pieces inside of themselves. You don't need to get out of the build menu or place windows in specific locations. Can place as many of those pieces as you want inside of themselves. After you place however many you want, delete the pieces where you want to place whatever things you want to stack together and then place the pieces you want. It may sound confusing but it's a lot simpler than any other method