Sharing an n-guardring with the nwell practically is (nearly) the same as putting nwell-taps with minimum spacing at the periphery of the nwell, which is a smart practice anyway and very much recommended for good shielding. If you want even better shielding, put a p-guardring around the nwell with (at least) minimum spacing.
An extra (minimum spacing) n-guardring around the nwell doesn't make sense, in contrary could be detrimental as to build another parasitic npn BJT with the nwell.