i'm not sure what you mean by there is no split on flat geometries. when you have a smoothingroup split you HAVE to use a uv split, you don't have to do this the other way around tho. i can only guess but unless blender and xnormal are synched there will always be some issues, slap on a diffuse map and specular and…
I mean that I split only edges (on the model) that are marked as sharp. And I don't know what do you mean in your second sentence :D Here it is how I'm doing that: - flatten mapping my UV and then stitching pieces - select hard edges on the model and mark them as sharp - apply Edge Split modifier with checked "sharp edges"…