Follow normal privacy post setting instructions for post A... then 

Before you begin installing the rail in to the bottom rail of each post, be sure to tape over the rails to prevent concrete seepage...then build the section and normal set Post B.   

Once this is complete...fill both posts with concrete to 24" above the ground.   

Then using #10 pan head screws 1" long (white matching caps available), screw in each tongue and groove section to the top rail and the bottom rail approximately 1 inch into the top and bottom rail.  Then also screw in the top rail on one side on the inside of the post. (34 screws per section)