I use both M's and Nikon Rf's. The S2 is my 50mm dedicated camera, better 1:1 finder than just about every rangefinder made - only rival today is the Bessa R3 and Zeiss ZM's.
However - if I was buying today, I would go for a S3 - you have 35/50/105 finder built in ( a bit crowded - by no worse than a M6/MP). This way you can use one body three lenses and alternate between the M's and the Nikon's.
I agree that using both system at the same time is a bit of a pain.
One reason for getting the Nikon system is that the old Nikon lenses are very good. They are in many ways better than the Leica lenses from the same era (35f2.5 - versus Summicron v1 - the Nikon is better!!), The 50f1.4 Nikon's (better than the Summilux 50f1.4 v1 - and rivals the vII too) and the classic 105f2.5 (one of the best portrait/landscape lenses ever made - even by todays standard).
The S3 has dropped in price considerably over the last couple of years - the introduction of the Millennium S3 caused it. For just a couple of $100 more - you can pick up a S3 - and as with most Nikon's it comes with a lens - usually 50f1.4'.
A 35f2.5 runs anywhere from $250-350 and the 105f2.5 around $400-450.
In short, you can get a 35f50/105 and a S3 for what a late Summicron 50f2 costs now (when did these go to $1300-1500 anyway? - they used to be $500-600 - worth that, but $1300 - no way for a 60 year old design!).
Beware though - Nikons are very habit forming!
Nikons also tend to work well. rarely need service - occasional cleaning, but of my 14-15 Nikon Rf's - only two needed service in the last 3 years. A S4 had the curtain come off and one my SP's developed a problem with the film rewind ( total cost about $300).
Better track record than my M's.