I think technically you could just do it on the 2.4 (the 2.4 radio is actually doing all the deciding - whether to send de-auths, withold probes, etc), but we're recommending setting both radios identically.
Toggling airplane mode might help the clients that tend to stick to 2.4, and also have them forget the 2.4 wlans (if you have any with distinct SSID)