How are multiple frequencies monitored at once?
First let's get rid of some misunderstandings:


Many smaller aircraft have just one single radio.
Being told to monitor a frequency does not mean you have to listen to two frequencies at once. See: Being told to monitor a frequency, when are you allowed to transmit on that frequency? Any controlled flight will always be under the control of exactly one controller, never more at the same time.


Now, as for actually listening to multiple radios at once, that is certainly possible, and quite normal on large airliners. In the cockpit of an airliner there will often be up to somewhere around 10 different audio inputs, each with their own volume control. This can include 2-3 VHF radio, an HF radio, NAV radios, communication with ground crew (pushback/handling), as well as audio from the cabin (the PA) and probably a few others. All these audio streams are merged into one, based on the individual volume controls.

Recently having had the opportunity to join a crew on a flight on the jumpseat, I was quite amazed at how good the pilots were at distinguishing between the multiple audio inputs, even when different people spoke at once. But with practise, it's actually not hard to see how it's done. Each audio channel will have a different "topic", using different key phrases and words. An ATC ground unit will be talking about taxiing, whereas the tower will be talking about taking off and landing. And the cabin PA will be explaining how to fasten your seatbelt to a bunch of passengers not paying attention. And of course then there is the fact that people all have different voices. Sometimes it is easy (a female on one frequency and a male on another) and sometimes it is more tricky, but I have never heard two people with the exact same voice.