Changing the number of columns in archives

Hi,

I am looking at modifying the Oblique theme for my own personal site, and am creating a personal child theme for it so that any changes aren’t removed should any updates happen. I want to add in a portfolio with a custom archive and single page. In the typical post archive page, you have a maximum three columns of posts. How would I modify this, for my archive-portfolio.php page so that there is a maximum of two columns of posts instead?