Thanks - glad you like the site.
Single column/two column layout is really defined at the template level. We’re using Structure, so content editors can just select the template that they want to use to render a piece of content.
We’ve got a standard two column template, a one column template for things like the timeline page, but we’ve also got a kind of hybrid template that they can use as a kind of “Section homepage” (e.g. the Building the Industry page) that’s a mixture of one and two column. Interestingly, all of these templates display the same content type/channel. So when a content ed is creating a piece of content, they define what they want to display around about it - for example, they pick an entry or two to display in the right hand column, or they can elect to display a collection of links, or they can display links to other pages within a given section, etc.
There’s a wee downside to this approach in that some fields for a given piece of content don’t really do anything when that content type is applied with a certain template (e.g. right hand column configuration when they use a single column template). Some mechanism of greying out some of the custom fields when a certain template is selected would be a nice-to-have. But overall I think it’s working well.