Talking through the firewall

Hallo guys,

I'vo got the problem that I have to sync two computers one one network
with two computers on an oter network. Between these networks there is
the linux firewall with DHCP-Service only to one interface.
I know how to configure the IPTables to let one computer in network A
talk to one computer in network B. But whats they way to let computer
1 and 2 form network A talk to computer 3 and 4 in network B. Maybe it
is not possible. Does anyone knows if it is possible or better what
can i do to fix the problem?
(The computers are all running W2k)
Thanx for any answer
Juergen