I’ve set my blog layout to masonry with 2 columns in customizer. However, only 1 column is showing up as you can see here: https://www.goalsarecool.com/blog/
That’s a mobile responsive behavior of the theme and is intentional.
Modifying the responsive feature of X is not easy and we cannot support the customization because there are lots of variables that needs to be taken into account to make everything work correctly.