In order to regulate the air pressure, the "breathing hole," as it's officially called, cabin pressure during flight is applied only to the outer pane.

In order to regulate the air pressure, the "breathing hole," as it's officially called, acts “as a bleed valve, allowing pressure between the air in the passenger cabin and the air between the outer and middle panes to equilibrate.”The hole serves as a bleed valve, allowing pressure between the air in the passenger cabin and the air between the outer and middle panes to equilibrate. This tiny little hole ensures that cabin pressure during flight is applied only to the outer pane.