I had contemplated that approach, but I didn't want to 'misrepresent' the fact that the second payment was not contemporaneous with the first payment (which had been timely submitted).  Yours is a simple solution and I had considered it, but then I got to wondering why I couldn't find a field for this type of additional payment.   I do like the suggestion.  The IRS surely is keeping track of the timing of payments remitted and will calculate penalties accordingly.