The Two Towers
Symbolic towers get a bad rap. There's the Tower of Babel—no good. There's the Ivory Tower—a dismissive term for academia. And then there are all those towers that Disney princesses—from Cinderella to Rapunzel—are constantly getting locked inside.
Poor towers. What have they ever done?
Well, in The Two Towers, they've done a whole lot of bad:
"Who now has the strength to stand against the armies of Isengard and Mordor? To stand against the might of Sauron and Saruman and the union of the two towers?"
Barad-dûr and Orthanc are very much alike. They are tall towers, built to be defended in times of war (Barad-dûr by Sauron and Orthanc by the Dúnedain), and they harbor great evil. Yes, Orthanc doesn't have a giant red eye of fire, but it does have the ever-watchful Saruman and his Palantír.
But why towers? Can't it just be The Two Strongholds, or The Two Fortresses, or The Two… Places of… Evil People. (Maybe that last one doesn't really work.) The point is that it might be more than alliteration that drew Tolkien to the "tower" idea. There's something symbolic about a tower. Why do people build such enormous skyscrapers? It's not just for earth surface area efficiency; it's to make a statement.
A tower is a seat of command. Both Saruman and Sauron use it as a watchtower of sorts, surveying both the building of their dominions and the comings and goings of their surrounding enemies. A tower is a place of power and prestige: something not just for practical purposes but also for admiration. And okay, the alliteration sounds pretty nice too.