Basically change the LAN/Router IP on the 2nd router to +1 of your main router. So like the post above, main router = 192.168.1.1 and the 2nd router = 192.168.1.2.
Then disable DHCP on the 2nd router. This way any devices connected to it will pull the IPs from the main router and everything will be able to communicate with eachother.
And to wire it up, you'll want to avoid using the WAN port on the 2nd router (unless you change the defaults in the configuration, which I'm assuming Tomato can do). Just use the LAN ports only, so R1-LAN1 connects to R2-LAN1.