First of all, if I take for example a Dual-Band N router with two 300Mbps bands.
Dual-band means that it operates in both the 2.4 and 5 GHz Wi-Fi frequency bands. I think you're talking about link rate.

Each band SHOULD have it's own SSID. If you only see one, it might be that your wireless NIC doesn't support 5GHz.
http://www.smallnetbuilder.com/wirel...outer-reviewed
The 2.4 GHz radio's default SSID is ASUS and the 5 GHz radio is set to ASUS_5G.
Using non-N clients can slow down your network, but the router shouldn't have to slow down all clients to b or g speeds. You would need to get 802.11ac wifi cards in order to get ac speeds. Otherwise, you will be limited to the max of your wifi cards.