Complex sentence: The eight Turtles head below the ground to track down the Technodrome, but their presence together only clues Shredder into the fact that there may be a version of himself on this world that he can ally with to take the Turtles down.
Split into simpler sentences: The eight Turtles head below the ground to track down the Technodrome, but they draw the attention of the Shredder. Quickly putting the pieces together, the Shredder realizes that there may be a version of himself on this world that he can ally with to take the Turtles down.