Although different styles of flying will use dual Rxs for different reasons, In our 3D/IMAC birds there are primarily 2 ,
1 was to handle the high currents of multiple servos. However, if you do a search, it has been shown that a single Rx could handle as many as 60 high torque digitals without failing.

The 2nd reason and the reason that I ran 2rxs' is for redundancy in case one fails. This theory has been strongly debated. I have recently removed my second Rx.