Which towers are the Two Towers?

[Manwe Sulimo]

In the end of The Fellowship of the Ring paperback version (or one of them, at least) it says this:

Here ends the first part of the history of the War of the Ring.
The second part is called THE TWO TOWERS, since the events recounted in it are dominated by ORTHANC, the citadel of Saruman, and the fortress of MINAS MORGUL that guards the secret entrance to Mordor; it tells of the deeds and perils of all the members of the now sundered fellowship, until the coming of the Great Darkness.
The third part tells of the last defence against the Shadow, and the end of the mission of the Ring-bearer in THE RETURN OF THE KING.

Hope that answers your question.