technically speaking there is not a single problem doing what you are planning.
We are even planning a small nanocase to mount two and three NS1 a la buchla :)))
Anyway, having two nanosynth puts you in the perspective of owning a 50 modules modular… This sums up to a lot of potential and, along with the digital waveforms that you can output with the Arduino, a pretty good polytimbral system! (with the MOZZI libraries you could also put some drum sounds in one of the two NS1…)…
Regarding the MIDI, natively is coming from the USB but the serial pins to use the ARduinoMIDI libs are there so it would be pretty simple to add this, along with the usual MIDI din connector, photocoupler and some more.
Also,if you want to start playing with components, have a look at the experimenter’s kit! 🙂
Let me know!