Unfortunately if I fix the issue in the error report i.e. take this line out so the z coord does not have different points the cubes still won't merge! Perhaps it would be better to define the two cubes independently with the necessary scalar coords e.g. 
```python
cube_1 = iris.cube.Cube([1, 2])
cube_1.add_aux_coord(iris.coords.AuxCoord(0, long_name='x'))
cube_2 = iris.cube.Cube([3, 4])
cube_2.add_aux_coord(iris.coords.AuxCoord(1, long_name='x'))
cube_2.add_dim_coord(iris.coords.DimCoord([0, 1], long_name='z'), 0)
single_cube = iris.cube.CubeList([cube_1, cube_2]).merge_cube()
```