library of more than 2 3d objects

I understand that in a single scene Vuforia only has the capability of tracking two objects at once.  My question is that when we try to add 4 targets so that we can track 4 different object at different times it does not work.  My question is are we able to have a set of more than 2 objects to be tracked knowing that if the camera is looking at the object it will only activate no more than 2 effects at a time?