Disable the Snap to Grid (View > Snap to Grid) then go to Edit > Preferences > General and make sure that the Keyboard Increment is set at 1px. Select the second rectangle made in the previous step and make two copies in front (Control + C > Control + F > Control + F). Select the top copy and hit the down arrow once to move it 1px down. Reselect both copies, open the Pathfinder panel (Window > Pathfinder) and click on the Minus Front button. Fill the resulting shape with R=90 G=90 B=90.