Well, it appears that Tolkien himself was not wholly decided on the issue and preferred that it be left ambiguous. If pressed, however, it seems that he would have nominated Orthanc and the Tower of Cirith Ungol, although his original jacket design depicted Orthanc and Minas Morgul.
Here are some illuminating extracts from
The Letters on this issue:
Quote:
The Two Towers gets as near as possible to finding a title to cover the widely divergent Books 3 and 4; and can be left ambiguous — it might refer to Isengard and Barad-dûr, or to Minas Tirith and
B[arad-dûr]; or Isengard and Cirith Ungol.
Letter #140 (to Rayner Unwin, dated 17 August 1953)
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Quote:
I am not really 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.
Letter #143 (to Rayner Unwin, dated 22 January 1954)
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In the editorial notes, it states:
Quote:
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In his original design for the jacket of The Two Towers the towers are certainly Orthanc and Minas Morgul ... Minas Morgul is a white tower, with a thin waning moon above it, in reference to it's original name, Minas Ithil, the Tower of the Rising Moon.
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The first of the quotes above also touches upon the symbolic nature of the title pointed out by
Firefoot.
Since I do not have my books with me, the quotes above are provided courtesy of
Diamond18's post in the second of the threads linked to below, all of which provide further thoughts on this point:
Two Towers?
Which towers are the Two Towers?
Towers