After I created the above screenshot, I did discover a workaround which will serve its purpose.
I right-clicked within the cell I wanted to make two-column, and selected Insert > Insert Cells... then the "Shift cells right" radio button, which resulted in the 2nd column of cells (like "something something" and "an addendum to something" in my question's picture) being resized, with no 3rd column to the right of them.
So, I selected all the 2nd column cells and resized them to cover the width of the 2nd and 3rd columns.
Finally, I clicked the 2nd column of the row which I wanted to be two-column initially, and in the ribbon, selected Table Tools > Design > Borders > Right Border to hide the border of the cells.
Not the simplest, but it got the job done!