Thats what I normally do , but in this case the objects sharing the border are split and two different meshes, I split but before splitting they shared the same normal , after split the normals at borders diverged, I would like to preserve the original normal orientation before splitting, is that possible?
If the normals were averaged and didn't have any custom normals, you can average them manually on an already split mesh, you select both objects and add a edit normal to both then manually select pairs of the border verts and average selected, Recommended to not do that and just lock the normals before doing things that…