@emin, welcome to the community!
I'm curious, why do you wish to?
Also, it's not a problem that they have the same BSSID, as they're on different frequency bands (i.e. 2.4 GHz vs. 5.4 GHz)!
And to make the change, you add option macaddr 'xx:xx:xx:xx:xx:xx' to the respective config wifi-iface section.
See: Still need help changing my mac address? - #16 by lleachii