Waste heat recovery (WHR) systems based on the Organic Rankine Cycle (ORC) are a promising method to reduce the fuel consumption of heavy-duty diesel engines. This article considers a setup with two parallel evaporators and a hydraulically closed low-pressure part. This gives additional degrees of freedom compared to a pressure balanced low-pressure part and thus allows to increase the recovering efficiency. However, these additional degrees of freedom lead to a higher system complexity, which complicates the calculation of power maximizing steady-state operating points. To determine these power maximizing steady-state operating points, this article develops a model based optimization strategy. For this task, models from the literature are extended and validated by test bench measurements. It is shown that the model has a high accuracy both in steady-state and dynamic operating situations. The optimal steady-state operating points of the considered WHR system are compared with other system topologies, utilizing, e.g., a pressure balanced low-pressure part or a single evaporator. The results of this paper can be utilized as the basis for designing optimal control strategies for the considered WHR system.