When you are working in the container topic, Author-it rules prevent you from being able to modify the content of the embedded topic. When your cursor is inside the embedded content, the editing commands are disabled and the options on the Layout tab are not available. This is to ensure that the embedded topic is edited only at its source.
The following steps describe what to do if you already have a single table made up of multiple embedded rows from different sources, and need to split these to insert a line of text between the two tables.