Parallel-coupled dual-racetrack silicon microresonators can potentially be used for quadrature amplitude modulation. We analyze the evolution of the coverage of coherent output states of devices with varying device parameters. As the coupling constant increases, the coverage of coherent states initially improves then degrades, which is unexpected based on a prior preference for overcoupling. Increasing the quality factor generally improves the coverage. The influence of the refractive index modulation is found to saturate after reaching a certain level. Analytic formulas are developed to provide insight into the coverage evolution. These results are fairly robust against a small asymmetry of device parameters.