An AP servicing ssid on 2 channels at the same time would require 2 radios. It's like a walkie-talkie that cannot listen to 2 channels at the same time, you have to select one.
So APs usually have a radio in the 5ghz band and one in the 2.4ghz band. So you can have the SSIDs both on 2 and 5ghz bands or one band only.
But you cannot spread the SSIDs between channel of a same band.
Nicolas