Dual-modal positron emission tomography and computed tomography (PET-CT) imaging enables the visualization of functional structures (PET) within human bodies in the spatial context of their anatomical (CT) counterparts, and is providing unprecedented capabilities in understanding diseases. However, the need to access and assimilate the two volumes simultaneously has raised new visualization challenges. In typical dual-modal visualization, the transfer functions for the two volumes are designed in isolation with the resulting volumes being fused. Unfortunately, such transfer function design fails to exploit the correlation that exists between the two volumes. In this study, we propose a dual-modal visualization method where we employ 'visibility' metrics to provide interactive visual feedback regarding the occlusion caused by the first volume on the second volume and vice versa. We further introduce a region of interest (ROI) function that allows visibility analyses to be restricted to subsection of the volume. We demonstrate the new visualization enabled by our proposed dual-modal visibility metrics using clinical whole-body PET-CT studies of various diseases.