What you do is you either build 2 posts on each side of each rail then the rail can sit in between the two posts..then you don't have to worry about it moving..especially if you concrete them in. Think of fence post..concrete..then set your posts..then put rail between. No more worrying of it moving away. Then you have wood to start planking it together with now that you have posts. You would need 8 posts total for 2 rails.