You can add more than one face tracker to a scene, to create an effect that responds to more than one face.
The first face tracker added will track the first face that's detected by default. You'll need to assign a face to every other face tracker:
For example, set Tracked face to Face 2 for a face tracker to track the second face that's detected.
Adding more face trackers can have an impact on performance, so it's good idea test your effect as you build it using the Spark AR Player app.