How to count a word covering both triple and double word cells?
According to the rules:


  Premium Word Squares: The score for an entire word is doubled when one
  of its letters is placed on a pink square: it is tripled when one of
  its letters is placed on a red square. Include premiums for double or
  triple letter values, if any, before doubling or tripling the word
  score. If a word is formed that covers two premium word squares, the
  score is doubled and then re-doubled (4 times the letter count), or
  tripled and then re-tripled (9 times the letter count).* NOTE: the
  center square is a pink square, which doubles the score for the first
  word.


Which means:


If the word (or the extention of a word) covers two double word squares, the value of the word counts 4 times. 
If the word (or the extention of a word) covers two tripple word squares, the value of the word counts 9 times.


Note 1: The 2x and 3x only count once, in the turn they are covered.

Note 2: On the standard board, there are no 2x and 3x fields in a single row or column, so it is not possible to have a 2x and a 3x in the same word. And yes the start field (2xword) is in line with (3xword) but there are 8 tiles needed so it is not possible in a single turn. If there was a board containing these fields it would be logical to asume that they count for 6. (