There is no configuration difference at this point. I think there was one initially ages ago. The only difference between MMR and mirror-mode MMR is that in the latter case, writes only go to a single master instead of any master in the cluster.
"Writes only go to one master in the cluster." What happens if they don't? If I DNS round-robin a pair of systems and MirrorMode them, what happens?
In other words, if N-Way supports two masters writing while MM does not, how can it not have a different configuration?
I seem to remember MirrorMode being very easy to set up yet N-Way taking significantly more time but looking back at the docs I think that's just a matter of the syncrepl setups taking a bit of effort.
John