Just to clarify, the 2.4GHz doesn't have to be a G only band. It can be 802.11b/g/n, just so long as it supports G devices. Also, the iPhone 4S only supports 2.4GHz, so if you want it to connect at N speeds you'll definitely need your 2.4GHz network running as b/g/n compatible. And I recommend giving the 5GHz band a unique SSID. You can do that in the Advanced section in AirPort Utility's Wireless tab.
Any way to not broadcast the 2.4?