In my eyes, two towers are Isengard, the tower of Saruman the White, and Barad Dur, the fortress of Sauron the Darklord. Actually, in the movie trailer of "Two Towers", the names of these two towers are mentioned by Galadriel.

However, someone told me that one of the two towers should be Cirith Ungol. And this is the official answer from the author himself, as mentioned in Tolkien's "Letters of JRR Tolkien."

This made me quite puzzled. Indeed, one most important part of Frodo & Sam's adventure in "Two Towers" is how to pass Cirith Ungol. But put it in the context of the whole story, I feel Barad Dur should be the parellel of Isengard rather than Cirith Ungol.

Any comments?

                   在TORN的Green Book上好像看到过，说LOTR在出版之前并非三部六本，只是由于篇幅过长，被出版商划分成现在的三部曲。这其中包括书名、每一本书开头和末尾的总结和预告性文字都是出版商自行添加上去的，为此托老好像还很不高兴。所以关于“双塔”到底是哪两座，并没有给出一个明确的说明。按照故事情节，河西的故事最后集中到Orthanc塔，河东的故事最后到了Cirith Ungol，所以双塔一般来说就被认为是这两座。

            Indeed the two towers refer to Cirith Ungol and Othanc. One can draw that conclusion immediately after reading the book.
But if you have seen the movie trailer, it is likly that you become confused,for the movie producers had obviously msunderstood the original meaning of the two towers.