There is a top and bottom to attach the post to a single wall. If you were attaching the railing on each side to walls then you would need two kits.
This will work great in this application.
You mount the hardware first onto where the railing will go. It will clip the rail into place over t he screwed in mount. Then you will snap the top and bottom rails into place. We have a video on ariarailing.com
JJ-the short answer is yes these will attach to the existing posts. However, there is hardware included in each 6' and 8' panel and you can use that hardware to attach to deck posts.