Headphones do not perform any RF signal detection. Speakers or headphones are provided electrical charge from the output stage of a scanner AFTER RF detection. This electrical charge in the headphone causes an audible sound...a frequency that your ears can hear.
A headphone, earphone or speaker have nothing to do with detecting an RF signal or scanning channel. Any output device...speaker or headphone ... is acceptable. It can be a singular speaker..monaural or dual...to a stereo headphone but the audio has no need to be stereo. A single earphone would be fine.
Whether a scanner detects multiple frequencies is hardly relevant. All that matters is that you can hear it - and one monaural speaker is appropriate.
The signal detection is performed in a one stage of scanner (RF). After the RF signal's detection within the scanner it turns into an audio output. No musical material is there...just voice...so using a single headphone is appropriate.