In Chapter 25 of To Kill a Mockingbird, what does Bob Ewell mean by Tom's death being "one down...two more to go."

We learn early on that Bob Ewell is not a very nice man. He is vindictive and wants to make those pay for things that have happened to him. He blames Atticus for all of his problems, and he knows the only way to make Atticus pay is to go after Jem and Scout.

Now that Tom is dead, Bob knows that the only two left to deal with are Jem and Scout. He knows that by hurting and even killing them, this is the only way to really make Atticus pay. Bob feels like Atticus is the one for all of his problems. He sees that the only way to get Atticus out of the way is to take away what matters most to him, which is his children. He attacks Jem and Scout after the Halloween pageant and nearly kills them. Boo saves Jem and Scout that night and Atticus thanks Boo for his children. Atticus knows exactly what kind of man Bob is. 

When Bob says "One down... two to go" he is talking about getting rid of Jem and Scout. He has already taken care of Tom and feels that getting rid of Jem and Scout will teach Atticus his final lesson.