The Unimesh Vertex Data Merge merges the XYZ coords along with the bone assignments. Instead, use the
Unimesh Bone Tool to copy bone assignments from one vertex to all other selected vertices without otherwise affecting their position/normals/UV/etc. If the bone tool is not behaving for you, you can also use Extended Manual Edit and copy & paste the bone assignments one by one. You will need to do the vertices or groups that share the same bone assignment one by one, though - there's no way to just copy bone assignments wholesale from one mesh to the other.