^ This is correct- you can assign the Artifact in both places at the same time, but you won't receive the Showcase benefit if the Artifact is active in your Main/War Hall. It's a little subtle, but you can see the difference here - the Augustus Statue is active in the Main Hall, so in the Showcase it displays the Exhibit icon in the top right corner and the subtitle changes to "Assigned to Main Hall" instead of stating the active modifier. It also won't show up in the Active Benefits summary menu (the button in the bottom right corner of the Museum).