I am not at all happy about the title 'the Two Towers'. It must if there is any real reference in it to Vol II refer to Orthanc and the Tower of Cirith Ungol. But since there is so much made of the basic opposition of the Dark Tower and Minas Tirith, that seems very misleading. There is, of course, actually no real connecting link between Books III and IV, when cut off and presented separately as a volume.
decimate means reduce by a 10th