Here's How Dual Band Works.
(This is to the best of my knowledge, but I think I'm right because I went through the same thing)
I hope this helps answer your questions. Read through the facts and then I'll explain below.
FACTS
The new Apple Airport Extreme Base Station and Time Capsule are both capable of "dual networks."
Here is what you need to know/understand about dual band networks:
1. Dual band networks are TWO SEPARATE networks.
2. Devices on the first network
Here is what you need to know/understand about network speeds:
1. "n" is the fastest, followed by "g", then "b", then "a"
2. Networks only operate at the lowest common denominator - hence - If you have an "n network" and a device with "b" speeds jumps on it, your entire network speeds drop down to "b" speeds.
EXPLANATIONS:
Every Apple computer can now handle "n" speeds and all of their Airport line can handle "n" speeds. However, once another device that isn't capable of "n" hops on, the entire network drops down. The device can be anything from another Airport (such as an older Airport Express that caps out at "g"), to a computer, to a cell phone with WiFi (such as the iPhone itself - which can not do "n" yet).
To combat this, you can set up a dual network.
The main network is capable of "n". As long as you only connect "n" devices to it, you should maintain "n" speed capabilities. The main network can be linked up with other devices such as an Airport Express or another Airport Extreme/Time Capsule.
The second/guest network CAN NOT DO ''n". It caps out at "g". This network can be used for guests, or computers that are not capable of "n" so that your main network isn't slowed down. The second network also can not be set up with another device, such as an airport express, like the main network can be.
So, yes, you can choose what signals you want each network to emit and each option has its advantages and disadvantages.
For example, I have an iPhone with the Remote App that allows me to control iTunes wirelessly from my phone. I have a couple Airport Expresses hooked up on my main network (because that's the only place I can hook them up - they won't go on the guest network) and whenever I want to use the Remote App I have to enable WiFi on my phone and drop down the entire network speeds. If my main network was only "n" then I couldn't even get my phone onto it to use the app.
Hope that answers everything.