Wikipedia page for The Two Towers
The Lord of the Rings is composed of 6 "books", aside from an introduction, a prologue and 6 appendices. The novel was originally published as 3 separate volumes due to post-World War II paper shortages and size and price considerations. The Two Towers covers Books III and IV.
Tolkien wrote, "The Two Towers gets as near as possible to finding a title to cover the widely divergent Books 3 & 4; and can be left ambiguous." At this stage he planned to title the individual books. The proposed title for Book III was The Treason of Isengard. Book IV was titled The Journey of the Ringbearers or The Ring Goes East. The titles The Treason of Isengard and The Ring Goes East were used in the Millenium edition.
A note at the end of The Fellowship of the Ring and Tolkien's final illustration of the towers gives the pair as Minas Morgul and Orthanc. However, in a letter to Rayner Unwin, Tolkien instead gives Orthanc and the Tower of Cirith Ungol, but felt such an identification was misleading due to the opposition between Barad-dûr and Minas Tirith. Loosely, any pair from the set of five towers in the story could fit the title: the tower of Cirith Ungol (Cirith Ungol being a pass), Orthanc, Minas Tirith, Barad-dûr and Minas Morgul.
However ambiguous the title may be in the book, director Peter Jackson's film adaptation of The Two Towers designates the title as referring to the towers of Barad-dûr ...