Before starting, we can prepare the document.
If there are no rulers to the top and left of the page, go to View > Show/Hide > Rulers. Now click on the top ruler and, holding the mouse button down, drag a guide to the halfway point on the page, five and a half inches in my case. This will represent the fold line of the card.
Now go to Layer > Layers... to open the Layers palette and click on Layer 1 and rename it Outside. Then click the + button and name the new layer Inside. Now click on the eye button next to the Inside layer to hide it and click on the Outside layer to select it.