It is called forwarding. You need a router between them, which knows the routes to each of those networks (if it's directly connected to them, it knows the routes automatically). Also the client PCs default gateway (router also) needs to know the way (route) to them (via another router). If they are both connected to the same router, then they should see eachother fine without any other devices. If they are on separate routers, and each has an unused interface, you can connect the routers, and set up routes for the networks.
If you have more networks, and need the routers to be able to set up routes dynamically, you also need one of the routing protocols (RIP, OSPF, IS-IS,...).
You don't need iptables for routes and basic forwarding. You'd need iptables for NAT or firewall.
If you want a better answer, you should explain more (IPs with netmasks, and the connection scheme, what is connected to what and how, etc.).