This is how you can use your short or long range radio and listen to two different channels at once. 
Let’s pretend you are in Alpha 1, your team is on Channel 1, and command is on Frequency 50 as always. 

Now we will pick an Alternative Channel so we can talk to command, let’s just pick Channel 2 for now. Change the Frequency on Channel 2 to 50, set the frequency and now set it as the Alternative Channel. On the radios graphical readout you will see that “C2” has now changed to “A2” indicating that this channel is now indeed the alternative one. 

Next we will set the channels to different ears so it’s not all a confusing mess. Since we already are on the Alternative channel (A2) we will simply press CTRL+Right Arrow to set it to your right ear. Next we go back to Channel 1 and press CTRL+Left Arrow to set it to our left ear. 

You will now hear your Team in your left ear and command in your right ear without changing channels. You can also talk to both channels separately without having to switch channels, It’s CAPS LOCK for Channel 1 and T for your alternative channel, which is Channel 2. 

You can switch your main channel around as much as you want, you will always hear your Alternative Channel in your right ear as long as you don’t designate another channel as the new Alternative channel. 

TL:DR: 

1) Go to a channel you wont use (usually channel 9) 
2) Set its frequency (Command, other lead elements, etc) 
3) Set the channel as your Alternative Channel. 
4) Set the channel to one of your ears 
5) Go back to your Teams Channel and set that to your other ear 
6) Now you listen to both channels and talk to your team with Caps Lock(CH1) , talk to Command(Alt Channel) with T 

You can switch them around however you wish of course, if you happen to be like me who can’t use the alternative channel hotkey for whatever reason, just switch your main channel to the alternative one with the numpad so you can talk on it, don’t forget to switch back after you are done talking though, otherwise you will not be able to hear your team since both your Main Channel and your Alternative Channel are the same. 

The above is an example for the Short range radios like the RF and 152. For the Long Range Radios (the backpack radios) everything is the same, except your Main Channel is on CTRL+Caps Lock and the Alternative Channel is on Y.