I'm not sure why you need to use the bridge interfaces. The configuration below should work.
/ip address add interface=ether1 address=192.168.1.1/24 add interface=ether2 address=192.168.2.1/24 /ip pool add name=pool1 ranges=192.168.1.10-192.168.1.254 add name=pool2 ranges=192.168.2.10-192.168.2.254 /ip dhcp-server add disabled=no authoritative=yes bootp-support=none name=dhcp1 interface=ether1 address-pool=pool1 add disabled=no authoritative=yes bootp-support=none name=dhcp2 interface=ether2 address-pool=pool2 /ip dhcp-server network add address=192.168.1.0/24 gateway=192.168.1.1 add address=192.168.2.0/24 gateway=192.168.2.1 /ip dhcp-client add disabled=no interface=WAN use-peer-dns=yes add-default-route=yes /ip firewall connection tracking set enabled=yes /ip firewall filter add chain=forward action=accept connection-state=established add chain=forward action=accept connection-state=related add chain=forward action=drop connection-state=invalid add chain=forward action=drop out-interface=!WAN src-address=192.168.1.0/24 comment="Prevent inter-subnet communication" add chain=forward action=drop out-interface=!WAN src-address=192.168.2.0/24 comment="Prevent inter-subnet communication" /ip firewall nat add chain=srcnat action=masquerade out-interface=WAN