2 Nights on Mont. St. Michel?

I think 2 nights is one too many. I would arrive at MSM as early as possible on day one and then spend the day and night and leave early on day two and visit St. Malo and then perhaps head on to your next stop e.g. maybe Bayeux. Most of a day is plenty. Or you could spend night two in St Malo to enjoy it in the evening.

1 on MSM and 1 in St. Malo would be ideal. St. Malo is a gorgeous town. I've stayed there twice and it's so lovely when the tide comes in in the evening while you're enjoying seafood. It's about time I get back there.

I agree with both other posters.. one night on MSM.. one night St Malo or...

I have been to MSM twice.. once decades ago.. and once this past summer. Staying the night is nice because the days in the season are really unpleasantly crowded.. so enjoying it after all the daytrippers are gone would be lovely ( I have not done that)

But there is not much to do there.. you can walk around entire area in an hour or two.. ( I suppose if you like touristy souvenir shops you could spend some time shopping.. I do not)
Enjoying the view.. the atmosphere .. and a nice meal overlooking the bay.. lovely.. but two nights.. no.. not that lovely .

this past summer I did just what RS suggests in his book. Showed up around 4pm, the crowds were thinning, went back later at nite to see it lit up, then back again in the morning and left around 11am. I day seemed sufficient. However I also prefer avoiding packing and moving every nite if I can. I visited Cancale and St. Malo on the way down to MSM from Dinan on that trip. If your wife just wants to walk around and perhaps have lunch or dinner, then a day trip from MSM to St. Malo would be preferable, IMHO, to changing rooms twice. And planning it this way gives you a bit of a hedge if the weather looks like it will be better one day than the other. The caveat, though, is that restaurants both on MSM and the immediate area close early, and there is no nite life. So sometimes you just have to suck it up and move 2 nites in a row.

On my trip I did 2+ days in the St. Malo area (I was in Dinan, not too far away), then did MSM as a 1-nite trip.

Ditto to previous posters' comments. We visited MSM in October 2015 and followed the recommendations from RS book, staying the night and departing the next morning. I can't imagine staying two nights in MSM--there just isn't that much there to do.