There are five possible towers: Barad-dur, Orthanc, Cirith Ungol, Minas Tirith, and Minas Morgul.  Tolkien himself in his letters (I don't have them with me at the moment, so I can't give the exact references) said that the title could remain ambiguous, but played with various combinations: Isengard & Barad-dur; Minas Tirith & Barad-dur; Isengard & Cirith Ungol, etc.  The problem arose because Allen & Unwin decided -- mostly for reasons of cost -- to publish the LotR (which is *one* novel in six books) in three volumes and asked Tolkien to name titles for each of the three volumes.  Tolkien came up with The Two Towers, but later said that he didn't think that any title would neatly cover Books III & IV.
     Peter Jackson has obviously decided to go for Isengard and Barad-dur: those are (very clearly) the two depicted on the poster and the two referred to in the various trailers thus far.

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All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
From the ashes a fire shall be woken,
A light from the shadows shall spring;
Renewed shall be blade that was broken,
The crownless again shall be king.