The page footer has two equal sized columns on the larger viewport. These columns stack when the viewport become narrower than 768 pixels.
The footer is split into two equal sized columns by using the col-md-6 classes. These columns get 50% (6 of 12 columns) of the space on the medium (md) and larger grids. As already mentioned, you can also choose to use Bootstrap's Sass mixins to build your grid.