It is simple to do this.
Login to the ZoneDirector with Command Line and go to Global Configuration Mode.
Enter to Load-Balancing Mode and enable the Radio Level Load-Balancing. It is disable by default.
Use wifi 0 for 2.4GHz and wifi 1 for 5GHz.
You may adjust threshold level if you wish to.
Check the below picture for your reference.
Here I show with Ruckus Zone Director 1106 and AP R710.
Zone Director Firmware is 188.8.131.52 build 27.
Have a good time.
(Be knowledgeable, pass it on then)