from the documentation...
i.e. if both sites are using 192.168.1.0/24 on the LAN, no site to
site VPN will work. This is not a limitation in m0n0wall, it's basic
IP routing. When any host on either of your networks tries to
communicate with 192.168.1.0/24, it will consider that host to be on
its local LAN and the packets will never reach m0n0wall to be passed
over the VPN connection. Similarly, if one site is using, for example,
192.168.0.0/16 and one using 192.168.1.0/24, these subnets are also
overlapping and a site to site VPN will not work.