Make a global variable name it for example :Camera
Make a invisible sprite and add a scroll to behavior to it.
Make events:
When N pressed - set Camera to 0
When M pressed - set Camera to 1
Camera = 0 - Set Sprite X to : lerp(self.X,Player1.X,1*dt)
------------------Set Sprite Y to : lerp(self.X,Player1.Y,1*dt)
Camera = 1 - Set Sprite X to : lerp(self.X,Player2.X,1*dt)
------------------Set Sprite Y to : lerp(self.X,Player2.Y,1*dt)