This is not true. the difference in frequencies can and will have a big impact on your throughput (amount of data sent received in the same time frame)
2.4ghz is a longer range lower throughput channel, and 5ghz shorter range higher throughput channel, 5ghz band will struggle to pass through solid objects ie walls.. so if your in same room or above below router 5ghz will be much faster in general than 2.4ghz. But if your in a room divided by brick or stone walls 5ghz will suffer quite badly. there are no devices on the market that can use both frequency's at the same time, your on 1 or the other as you choose, even if you connected your phone to both channels it would pick the 1st one it identified, most likely 5ghz 1st (due to aforementioned throughput) and as you went out of range it could then re scan and log you onto the 2.4ghz band. But you would need to reach the very limit of your routers range b4 it would change, causing major slowdown.
the oporational frequency is completely separate to the wifi draft ie a/b/g/n these letters simply determine the wifi link speed not frequency.
ssid = service set identifier. the numbers have nothing to do with frequency at all. There just a unique identity marker so u can differentiate between the many wifi signals in your area. much like your house number or ip address.