[UPDATE]: Source tab is now including in the latest version of Mingle. You will now be able to edit in a line break using the Source tab.
This is a temporary workaround due to a known issue that we are working to resolve in a future dot release. Stay tuned, but for now please follow these instructions.
When adding one or two column layouts, entering a line break before and after inserting a column layout will help avoid not being able to edit content above or below the column layout. If you add two consecutive column layouts, please insert a line break in between the two layouts if you intend to add content in between them.
The Mingle Team