This had been the back up plan he had come up with.
Atticus had divided each group equally and had tried to balance each of their strengths by fixing the Ravenstein youths according to their individual strengths.
Chubby had been close to the weakest amongst them back when they were at the Raven camp, so Atticus had fixed him with another Ravenstein youth who was stronger than him.
He had also done this with the others too making sure that the strength of each of the 8 units was more or less balanced.
What Atticus had planned for Aurora and Nate was exactly what he had termed it, backup. Those two were basically his trump cards in case anything goes wrong.
Atticus had no idea what his opponents were capable of, and he had no intention of underestimating anyone.
He was going to plan for every scenario he could think of and keep his guard up at all times.