From Zone Director think I can do this but you don't mention if you're using ZD for configuration.
In ZD I can configure an AP to specify one WLAN group to broadcast using 2.4GHz and another WLAN group to use 5GHz.
So I put/assign "tecnici" in one WLAN group and "guest" in the other.
Might help (or might not)! Perhaps someone else can explain more clearly. Image shows the WLAN group choice on the AP edit page. My school WLAN group is called "HARROWDEN" but I could create others and have them broadcast on seperate frequency.