Twin windows that are framed within a few inches of each other and that are covered by 4 panels can be configured with either open center panels or as double bi-fold. Both options look very similar when panels are closed.
In the open center configuration, the 2 panels in the center are hinged to a vertical t-post in the center of the frame.
Pro: Most common and strongest means of mounting shutter panels to the shutter’s frame.
Con: The 2 center panels cannot be swung open flat against the wall as with double bi-fold .
Shutters that are double bi-folded are built with 4 panels, with the 2 center panels hinged to the left and right-most panels.
Pro: Allows panels to be folded back against the wall for an unobstructed view through the twin windows.
Con: Panels may sag slightly (approximately 1/8 inch to 1/4 inch) when swung out of the frame and will have to be lifted back into place. This is because the panels are hinged to one another, rather than a t-post.