It's certainly possible. The Switch will have sold around 4.3m WW when ARMS launches barring glaring stock issues (this was calculated with a 150k global baseline and a stock boost for ARMS).
If Nintendo can maintain that it should be around 5m when Splatoon 2 launches. That is a big enough install base for a juggernout franchise to achieve 2m at launch, but I think the deciding factor will be japan (as well as stock).

Seeing how well it's tracking on COMG, Amazon.jp and Rakuten it might do 1m or close to that on launch in japan alone. 1m for EU+US+RoW should be achievable for a big game launch.
MK8D did 1.2 WW for launch, with around 300k of that coming from japan. That puts the rest around 900k and this was for a port, not a brand new game like Splatoon 2.

In short 2m launch is feasable. The remaining 2.7m legs within this year is where things get tricky, because they depend so much on Nintendo providing enough supply, especially in japan where the game will probably have it's best legs.