Yes that’s due to their custom CSS. Are you familiar with CSS, and do you know how to customize CSS? That’s how you can make customizations to the design of your site. For starters, you can change the width on line 1472 to around 40-44% and that would make it show 2 posts on each line….example –
.home-feature-bg-alt .featuredpage .page,
.home-feature-bg-alt .featuredpost .post {
float: left;
margin: 0;
overflow: hidden;
padding: 0 0 5px;
width: 44%;
}
If you’re trying to make it look exactly like Copyblogger’s design/look and feel, then you would probably need to hire a developer/designer to code this for you if you don’t already know how to customize CSS. It shouldn’t take too long since the example you’re trying to follow is pretty minimalist.