VPN

2 Networks are connected together with IPSEC VPN tunnel, ping, all functions work fine, only with DNS there is a problem. The Computers from one network cannot resolve the IP address for a computer in the other network. I tried to add the 2nd m0n0wall to the list of DNS servers in the 1st m0n0wall, add to alternative DNS server in DNS Forwarder Services to the other Domain, but it works not. The only solution was is to add every host with domain to a DNS Forwarder with fix IP, but it's not a flexible solution  .
Any idea, help?

This is a well-known routing issue. You fix it by a static route.