Usually, turning on the AC will turn on both fans. Anytime the secondary/AC fan (left side US) runs, the primary one (right side US) has to be running as well.
If the AC is not on, then the main radiator cooling fan will come on as needed - generally off while going at highway speeds, and on while idling.
As to why a fan isn't running, there are a few possibilities:
1. Cooling fan fuse/fusible link
2. Cooling fan relay
3. Fan control module
4. Mechanical failure of the fan assembly
Since you have a twin setup, you could troubleshoot #s 1-3 by switching the electrical components. #4 can be tested by safely (ignition off) spinning the fan by hand to see if it moves freely and sounds or feels normal compared to the other one.