Make sure you design your network right, if your allowing clients to connect on the 5GHz radio, as a large number of clients will slow your MESH layer down.
My advice would be to us the max-aid command which will limit the amount of clients that can connect to the 5GHz radio:
set max-aid wlan32 40
Note: I am not saying limit the value to 40, I am only providing you the script. 🙂You could also delve into the possibilities of using Client Admission Control (CAC) (if your using the SZ/SCG platform), but it is early days yet and there are loads of extra values needed to make this option viable in certain networks.
CAC works by looking at clients usage, airtime utilisation, client capabilities and client numbers. By assessing the values you put in it will either allow or deny it entry on to the radio based on positive or false positive values (you can configure seperate entry criteria on either of the AP's radios).