I don't see how adding a second router could provide any isolation.
This is how I setup 2 networks behind 1 cable modem:
Cable Modem (WAN IP address)
+==> Router0 (network0 = 192.168.0.xxx through NAT)
+==> 0.port 1 ==> device1 (192.168.0.1)
+==> 0.port 2 ==> device2 (192.168.0.2)
+==> 0.port 3 ==> device3 (192.168.0.3)
+==> 0.port 4 ==> router1 (network1= 192.168.1.xxx through NAT)
+==> 1.port 1 ==> device4 (192.168.1.1)
+==> 1.port 2 ==> device5 (192.168.1.2)
+==> 1.port 3 ==> device6 (192.168.1.3)
+==> 1.port 4 ==> router7 (192.168.1.4)
The network0 devices can't "see" the network1 devices and vice-versa. But both network0 and network1 can access the internet. If you want to feel more secure, you can put another router on one of the router0 ports and put the 2nd network on that router like so:
Cable Modem (WAN IP address)
+==> Router0 (network0 = 192.168.0.xxx through NAT)
+==> 0.port 1 ==> router1 (network1= 192.168.1.xxx through NAT)
+==> 1.port 1 ==> device1 (192.168.1.1)
+==> 1.port 2 ==> device2 (192.168.1.2)
+==> 1.port 3 ==> device3 (192.168.1.3)
+==> 1.port 4 ==> router4 (192.168.1.4)
+==> 0.port 2 ==> router2 (network2= 192.168.2.xxx through NAT)
+==> 2.port 1 ==> device5 (192.168.2.1)
+==> 2.port 2 ==> device6 (192.168.2.2)
+==> 2.port 3 ==> device7 (192.168.2.3)
+==> 2.port 4 ==> router8 (192.168.2.4)
+==> 0.port 2 ==> leave unconnected
+==> 0.port 3 ==> leave unconnected
The router solutions cost more than the A/B switch, but both networks have internet access simultaneously.