Do I need to set up the separate 5GHz on the AirPort Extreme.
Yes, if you want to match the Asus networks.
The setup of the AirPort is a bit tricky for some users, because you must first set up the AirPort to use only one name for both 2.4 GHz and 5 GHz networks and then later go back into the settings in AirPort Utility to adjust the settings again to match the names of the network that the Asus router is using.
Post back if you need a few tips on this.
Please note that WiFi devices may or may not automatically switch to the router with the strongest signal as they move about the house. Macs will usually do this pretty well. iPhones and iPads, not so much....but they are getting better about this.