Every device should have unique MAC address.
Of course you can just let them have the same MAC address, as long as they are not in the same subnet, not broadcasting wifi signal in the same area.

Just change:
/etc/config/network

For example, if you want to override the MAC of WAN, add in /etc/config/network

config ‘interface’ ‘wan’

option macaddr ‘xx:xx:xx:xx:xx:xx’

then restart