Tower of the Moon Minus Morgul
Tower of the Sun Minas Tirith
There is a interview with the bbc were Tolkien Himself talks about the title.
He has talked about it in many ways and the only thing clear is that he was not happy about the title because it is not clear.
What he said in the book itself certainly says "Orthanc and Minas Morgul"... but he also made different comments elsewhere as you can read in this thread.
With the differences of opinion shown above with just THIS one bit--just the title, of a book written just over 60 years ago--and the many interpretations there, I'll never believe anyone who says they have the BIBLE all figured out.
I think it was the two towers mentioned - orthanc and minas morgul (purely my interpretation of course).. but I enjoyed the idea that is was meant to contrast against the two trees of his older lore, and as much a narrative tool as anything.. two trees - peaceful, two towers - war. sort of thing.
I've always thought of THE two towers as being Orthanc and Barad-dur, mainly because they are true towers (at least the way I envisage them). To me the other two main contenders, Minas Tirith and Minas Morgul are cities which contain towers, not towers in themselves.
Another interesting debate is whether these two structures, Perrott's Folly and the Waterworks Tower (you'll need to google them as I don't know how to hyperlink on these forums), were the inspiration for some of the mighty towers in Middle-earth. In his teens Tolkien lived only a few miles from these particular Two Towers in Edgbaston, and would have seen them often. In an age before skyscrapers these imposing buildings would have dwarfed everything nearby, and may well have fueled a young Tolkien's fascination with towers.
I've always thought of THE two towers as being Orthanc and Barad-dur, mainly because they are true towers (at least the way I envisage them). To me the other two main contenders, Minas Tirith and Minas Morgul are cities which contain towers, not towers in themselves.
But "Minas" means "tower", of course. Tower of Witchcraft/Black Sorcery and Tower of the Guard. So the image of a tower play a major role for both cities.
I have never realy read Saruman as a defector to Saurons side, rather as a rival, given the interaction between Sarumans Uruks and the orcs of Mordor. Having a nazgul land on his roof after Pipin revealed himself in the palantir always seemed to be Sauron demonstrating who was the real dark lord and who was the imitator rather than evidence of cooperation or alliance for this reason I have never been comfortable with Turbines and PJs interpretation of the relationship beteen Orthanc and Barad Dur. For my own part I always thought the two Towers title infered a balance between the white of minas tirith and the black of Barad Dur like a chess set, but cant speak to what Tolkien intended it to mean.
I find your interpretation intriguing! And it makes a lot of sense. I had never really put much thought to comparing M. Tirith and Barad Dur, either.
To clarify, I only think Saruman was a defector to Sauron's side insofar as he is only trying to use Sauron's power to gain individual power (and Sauron's power as a cloak). What is significant to me is that many events revealed in the Two Towers happened because Saruman became an enemy. An enemy that is playing footsie with a much more sinister adversary.