If you change IP addresses of the wifi gateway (normally 192.168.1.1) to 192.168.3.1 and the internal network gateway (normally 192.268.0.1) to 192.168.2.1 on router 1, you can directly connect an ethernet cable into a port of the other router to create 2 networks each with its own internet gateway. While you are in the router setup check if RIP is active. Set it as “in only”.

Change the details in router1 first, connect them up then switch on the second router (the one with the unchanged IP). The router already on should then get a query signal down the ethernet cable, and should add the route to its routing table. (if RIP is on)

Wireless would then be 2 separate networks, each with its own network id and password. as printed on the router label or box they came in.

Each PC/Laptop could have a static IP allocated in the router, if you wanted, but dynamic allocation will also work.

Your wireless devices will report 2 wireless networks available when you first start them after installing both routers, so you need to know which id belongs to which router to enter the correct WPA/PSK.

If you change a channel on the wireless router it will not interfer with the other wireless network, most come set at channel 11, so change the other to 5 or less if you want, that will give you a good difference in the wifi operating frequencies. Just remember to check the channel the wifi devices are capable of operating at.

Default gateways are allocated by dhcp, and the routers should learn the port to send network traffic out.

All you need do then is try to ping each IP address to verify connection is good.

Eg.

192.168.1.1 Router 1
192.168.1.2 Router 2

192.168.1.3 Computer 1
192.168.1.1 Default gateway (Router 1)

192.168.1.4 Computer 2
192.168.1.2 Default gateway (Router 2)

Remember to turn off DHCP on both routers. These IP address’ need to be staticly assigned.