The problem is that if you connect RouterB to RouterA then it will be possible for people behind RouterB to see the network and machines behind RouterA because RouterB's WAN port is also on RouterA's network. You would need to put a rule in RouterB that would block it from seeing anything on RouterA's network besides the default gateway address. Another way to do it would be to have a 3rd router, say RouterC. So the only 2 things plugged into RouterA are RouterB and RouterC each with its own clients. RouterA would have no client on its own. A 3rd way would be to get a more pro router, or load an OS like DD-WRT on RouterA and then you could put the 2 networks on different VLAN's and not route between them so they are separate.