This study addresses an “U” shaped assembly line with walking workers and flexible worker assignments. Workers, rather than machines, are the constraining resource in a dual-resource production system. Assignment strategies for walking workers based on Takt Time or restricted time zones of overlap are compared under different production conditions, including processing time variability and worker efficiency conditions. Four strategies have been considered namely: dedicated zoning, relaxed zoning, float zoning, and overlapped zoning. Discrete event simulation is used to assess the performance of these strategies under the controlled release of jobs to the assembly line based on a Constant Work In Process policy. Results show that heterogeneous worker efficiencies strongly impact the performance of all strategies. However, this impact depends on the specific strategy. The study contributes to a better understanding of the behavior of walking workers’ strategies in “U” shaped assembly lines controlled by a Constant Work In Process policy.