Multi-segment continuum robots, that offer inherent compliance and distal dexterity, are suitable for deployment in minimally invasive surgical procedures. Cable-driven mechanism is commonly used in continuum surgical robots but could lead to inter-segment motion coupling in a multi-segment robot. In this paper, we present a coupled mechanics model for a two-segment notched continuum robot to analyze the coupled deflection in the proximal segment due to the distal cable force. The model has been developed for two different conditions in which the proximal segment is initially bent (general condition) and initially straight (special condition). It allows us to introduce a decoupled design methodology that systematically determines a stiffness parameter in each of the segments, based on the desired coupled bending angle and other design requirements. Using the method, we fabricated a decoupled notched continuum robot and evaluated the model accuracy compared with experimental data with mean errors of 0.57° and 0.61°, respectively for general and special conditions throughout the 90° distal segment bending angle. It was also shown in a demonstration in a maxillary sinus phantom that the distal segment was capable of independently perform omnidirectional steering without the proximal segment getting in contact with its surrounding nasal wall.