Solution
DIAdem does not allow table cells to be "merged" or "combined" in the way that is familiar when using Microsoft Excel.
To workaround this, it is necessary to:
- Split your table design into multiple tables. For the example above, it could look something like this:

- Align the edges of each table perfectly, using the Position tab in the table Settings.

- Group the tables together, so that they behave as one single table.
- This can be done by highlighting all tables, and clicking Group from the top toolbar.
