Sorry, forgot to answer the other half of the question.

No, it doesn't have to be left and right only. In fact, in my opinion, its best not to to help alleviate any adverse effects of only half the surfaces working. Here is how I setup the dual receivers in my planes.

Receiver 1 Receiver 2
Aileron Left Ail Right Ail
Elevator Right Elev Left Elev
Rud Rudder
Smoke Smoke
Kill Kill switch
Throttle Throttle

If receiver 1 fails, I have the right aileron and the left elevator. I fell this is better than having the left aileron and left elevator. Easier to counter any adverse differential motion.

Further, if either receiver fails, I have some control over the motor. I can either operate the throttle and kill the engine, or just kill the engine. Either way, there is some control. If the rudder fails, no biggie, just fly it (as long as its not hard over).

This setup also seems to balance the battery load, as the drain on the batteries always seems consistent to each other. The setup has been working in the last three 40% planes without any issues over the last few seasons.