The equipartitioning learning trajectory (Confrey, Maloney, Nguyen, Mojica & Myers, 2009) has been hypothesized and the proficiency levels have been validated through much prior work. This study solidifies understanding of the upper level of co-splitting, which has been redefined through further clinical interview work (Corley, Confrey & Nguen, 2012) in response to performances at that level on the field test (Pescosolido, 2010). In addition, it provides new perspective on the relationships between lower levels within the trajectory, and co-splitting and the upper levels. It describes a teaching experiment on co-splitting with sixteen rising fourth-, fifth-, and sixth-graders, and examines their development of that construct. The primary research questions focused on relating students' progress of development and strategies for co-splitting to the lower levels of the learning trajectory, their knowledge of multiplication and division, and the tasks and parameters used in the curriculum. The results were analyzed and synthesized with respect to the findings for each of the research questions. The study shows that all of the lower levels of the equipartitioning learning trajectory are utilized by students interacting with co-splitting to various degrees, and it is critical that a strong, conceptual understanding of multiplication and division be developed prior to, and in parallel with, co-splitting. The research establishes a precedent for making connections between levels, and between trajectories, in a learning trajectories based instruction (Sztajn, Confrey, Wilson & Edgington, 2012) model, and it also sets the stage for similar study of the other upper levels in the equipartitioning learning trajectory, such as equipartitioning multiple wholes and the distributivity property of equipartitioning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]