You set up multiple receivers up the same way as a single receiver, except you divide the functions. If you are using two channels for the ailerons, one channel for right and one for left, then one aileron gets plugged into one receiver and the other aileron gets plugged into the other receiver. The same with elevators and flaps (if you have multiple flap servos). Rudder is put on one of the receivers, throttle on the other, and other ancillary functions divided-up between them as you see fit.

You could drive two aileron servos by using channel 1 on both receivers, but you won't get any of the mixing features you get by using two channels. You'll still get the redundancy of one servo working if the other fails, though.

Don't put "ganged" servos on different receivers. That means if you have two servos driving a single control surface, you can't have one servo driven by one receiver and the other servo driven by the second receiver. This will cause a severe power drain if one servo fails.

Just make sure each receiver has an independent power supply and that its antennas are properly-routed.