Oh, my bad!
I was thinking of a horizontal application, where more vertical support must be added, using the masonry wall as a sort of foundation support, depending on how structurally sound that wall really is. In other words, @nhb80
would essentially be building a rough frame for a half wall, with top and bottom plates, using 2x4's at 16 inches on center, max.
For a vertical application, as you're thinking, the necessary support with the existing 4x2 top and bottom rails, should suffice, but adding another rail in-between, might not be a bad idea, for extra support.
Assuming the existing posts are structurally sound, and supported with the necessary hardware.
Of course, a full inspection of the existing material and the existing fasteners, that would remain for the project, is absolutely necessary, regardless of which application "N" chooses.
, Our century home has beaded tongue and groove boards on the ceiling of our front porch, where it's more protected from the elements. Honestly, I personally would not choose wood tongue and groove for fence panels. Not even dutch lap or ship lap due to exposure to the elements on both sides, which might result in warping, cupping, and/or twisting and the eventual unseating of the tongue and groove. Have you used this application in an unprotected outdoor location, specifically with wood material? If so, how has it held up?
I could see using composite material, for vertical tongue and groove fence panels, with an additional center horizontal rail or two, due to the weaker structure of composite material lends itself to bow more than solid wood.
I would love to know whether "N" has completed the project, and if so, what they did, and include photos of the finished project.