Unless you have a firewall or can put access control lists (ACL) on the router, then what you are looking at doing is not easily done since both network share a common network routing device. You would need something like a Smoothwall (open source) or Vyatta (open source) or Netscreen or Cisco PIX to firewall the segments from each other.
to continue the subnet structure from A to B, set B as follows
192.168.1.128/25 (255.255.255.128), add a static route from outside interface of router b to inside router a; point the clients to 192.168.1.128 for gateway. This will not provide load balancing for you, but should isolate the clients to their own side.
To answer your question ” So my question is what do I set side B router and clients ip/subnet/gateway to?” – the direct answer would be to set you gateway to 192.168.1.129 and make the hosts range from x.130-254
The router on side B should be configured as a gateway which will connect to its wan using DHCP to obtain its external address. It should have an internal IP of 192.168.2.1, and an IP range of 192.168.2.2 to 192.168.2.(2 + the number of connections you expect to have) on its own DHCP server. The subnet mask can be the same.
This should allow the two networks to be separated (invisible to each other, but still able to communicate), but still be able to communicate with the internet.
Additionally, you can set the Workgroups of the machines on each side to a different workgroup name for more security between the two sides.
This works on my network. My setup is shown below.
DIR-601(192.168.0.1 – connected to the internet)<—Wireless—>DIR-601(192.168.1.1)<—Ethernet—>Computer(192.168.1.2 – Able to access the internet, but can not see the network beyond the router it is connected to.)