How to gain access to another LAN?

Not sure if I stated the subject question correctly but here is what I want to accomplish:

I've got two routers running Tomato (Shibby). Each router has its own internet access via its respective WAN port.

Router A is the DHCP server for the following LAN: 10.0.2.0/24. Its LAN IP is 10.0.2.200.
Router B is the DHCP server for the following LAN: 10.0.3.0/24. Its LAN IP is 10.0.3.200.

I want the LAN clients connected to Router A to be able to see/access the LAN clients connected to Router B,...

How to gain access to another LAN?