If you have two viewports created on top of each other, it seems impossible to select a specific viewport (Civil 3D will automatically select the last active viewport or the larger one). The image below shows the active viewport is the plan view, if you want to activate the viewport for the setting out data table, you will not be able to achieve this by clicking with the mouse:
To activate the second viewport, click inside the plan view's viewport and press CTRL+R to toggle between the viewports:
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