The reason to have more than one scanner is because while a scanner my scan several channel, it can only listen to one at a time. So once it locks on a channel that is transmitting it is no longer scanning the other channels. This can cause you to miss transmissions you might have wanted to hear. For instance you have EMS, Fire and Law Enforcement for you local town programmed in, or multiple town in your area. The scanner my lock onto the EMS talking to the hospital for an incoming patient. While you are listening to that that Police may start a car chase at the same time. You would't know because the scanner was no longer "scanning", it was busy listening to the EMS transmission.
If you had more than one scanner running you could listen in real time to more than one system. In the above example one scanner would be transmitting the EMS, and the other scanner would hopefully pick up the Police chase you might have missed.
Or you could use one scanner to monitor conventional systems and another to monitor trunked systems. If you had enough scanners you may not even have to "scan" anymore, you could just set a scanner for specific agencies and monitor them.
I only have one scanner at the moment because of cost restrictions. I am poor and cannot afford multiple 3 or 4 hundred dollar scanners. I would like to get a cheap analog scanner for conventional system though. I am watching local flea markets and good will waiting to find a deal.
What I do have though is an SDR for my computer. Depending on what program you use you can scan with one, or listen to multiple frequencies at the same time. I let my 325p2 scanner do it what it does best and scan my area. And sometimes I use my SDR at the same time to just monitor the local police on one frequency and the sheriff on another. So I am essentially monitoring two channels, and whatever the scanner picks up.
All of these are run through my computer, even my scanner using the EZsoft virtual scanner control function. This way I can visually see what is happening and either mute or lower the volume on others if something interesting is happening on one.
And yes, it gets confusing, no doubt about that. Having the visuals helps me with that. And I have to sit and listen and pay attention. It is not something I can do while doing something else, like watching TV. If I do that I just use the one scanner. My brain simply can't take that much input at the same time lol
And I have no interest in rail roads or air traffic either. I only monitor Fire and Police for the most part. State police, my county sheriff, and my counties fire system.
Another advantage to multiple scanners is sometimes they just flat out miss signals. If you set two scanners next to each other with the same programming sometimes they will pick up different things. One might miss certain transmissions while the other catches them. It seems like the more systems and channels programmed into one scanner, the more you may miss. Providing you are scanning them all. Using multiple scanners you can scan smaller "chunks" with each one and improve your odds of not missing things. This is the reason I want a cheap analog scanner, so I can use one for conventional channels and the other for trunked systems.
There may be other reasons I am not aware of, like I said I don't really own multiple scanners. Maybe one of these guys with a whole bank of them can shed some light on why they felt that kind of an investment was worth it to them.