how do I make this a dual mailbox post like the last picture in the description shows?

The cross arm is "centered" into the main upright post and positioned parallel to the street. Then two mounting boards (purchased separately) are installed to both the left and right side of the post. We recommend the treated pine mounting board (Model# MBMBT15N). This mounting board includes self tapping screws that will attach mounting board directly to the mailbox post cross arm.