You need to put the new single radio APs in their own AP-GROUP.
Once you do that, you will be able to specifically set the radios to always be 5 GHz (aka 'a band')
Ensure you set the following in a NEW PROFILE for the NEW GROUP (i would not modify default-profiles...its too confusing later):
a) Under 802.11a radio profile .. 'radio enable' == enable (checked)
b) Under 802.11g radio profile .. 'radio enable' == disable (unchecked)
c) Under AP system profile ... RF Band == a
That last setting may be missing as-is.
Also, you may choose to set a NEW Virtual AP profile for this special/single SSID as follows:
d) RF Allowed Band == "a"
With all four of these settings all single radios you add to this new AP group have nooooo choice but to use 5GHz.
JF