Mobile Site Layout

Hi there, I have a quick question regarding the responsive design of my mobile site. What is the best way to edit where and how elements are laid out on the mobile site without changing how they appear on the desktop site? I’m very happy with how my site is laid out in the desktop version, but I’d like to make a few tweaks on the mobile side.

In the attached screenshot, you’ll see that I have an “Our Work” section and a “Testimonials” section. On a desktop browser, they display side by side and look great.

The next two pictures show how these sections display on my mobile device. I am able to tell the “Our Work” section to only display one item instead of two, but the “Testionials” section still tries to display right beside it. This makes two very narrow columns that aren’t very appealing. The actual testimonial content gets pushed down below the two titles. What I would like to do is force each section to display as one column with the Work section above the Testimonials section.