@Mitch Bailey (and @Jianwei Jia): MiM caps are not permutable because there is a significant difference in parasitic capacitance between the top plate (which is shielded by the bottom plate) and the bottom plate. If you use the dual MiM caps together, then you have a central plate which is the lower MiM cap top and the upper MiM cap bottom which is shielded on both sides. Making it non-permutable means that the analog designer is encouraged to pay attention to where the parasitics are.