Try this slight variation on what you have already done.
Detail line trap as you already have it.
Append trap as you have it - the same as the detail so that it always resets for each new detail line - BE SURE TO MAKE THE SAMPLE JUST A SINGLE LINE, NOT 2 LINES.
Paint a field for the entire width of the page (or the max width of your sometime second line if that is smaller.) Make the field a multiline field using 'End on' "None of the Above" in the advanced field properties.
Check your Table results. Your Append field should have 'full width' text from either 1 or 2 lines. (Be sure to make the displayed width wide enough to see the second line included or use the "Show Field Contents" option from the "Window" menu.
Create a calculated field using the TEXTLINE() function and indicate you only want line 2 of the multi-line field. Now you should be close to what you need if you hide the original append field. Lines with no second line will return an empty field.
If you need the contents of the second line split into individual fields just slice and dice the result of the textline separation using the most appropriate functions based on the line structure or content. More suggestions available for this if you are not alreay familiar with slice and dice - I can't remember if it is something that you have covered before.