Hi,
you can enable either the 5GHz and/or the 2.4GHz on a per AP basis in the RF Profiles.
See it in the documentation:
https://documentation.meraki.com/MR/Radio_Settings/RF_Profiles
I am not sure if this is also possible in the MX67W as I do not have such an MX. But i would expect it.
So on a per AP basis you can do it.
In your case you would need to go on a per SSID, where you can choose either dual band or 5GHZ only.
So I would try to implement one SSID with only 5GHZ.
The second I would try to configure dual band and just unassign all the channels of the 5GHz.
I am not sure if this will work...but I would try to do it this way.
hope it helps