Nef Kho,
I had something like that set up a few months ago to test a new network configuration. If I recall correctly, this is how I did it.
Your two network charts seem to have networks A and B reversed. To be clear, this is the reference for what follows:
Network A devices --- Network A Router --- Network B Router --- SME Server --- Internet Connection
Router A:
Set the local network IP address to 192.168.1.1 and have DHCP enabled to allocate some set of addresses for Network A in the 192.168.1.xxx range.
Set the WAN access to an address on the Router B LAN (e.g., 192.168.0.201).
Plug the WAN port of Network A Router into a LAN port of Network B router.
Router B:
Do not plug anything into the WAN port of Router B.
Set the local network IP address to 192.168.0.1 and disable DHCP (assuming the SME box will provide DHCP services for this network).
Plug a cable from the local interface on the SME box into any LAN port on Router B along with the cable from Router A and any other devices on Network B.
SME Box:
Set the Local IP address to something unassigned in the Network B range (e.g., 192.168.0.254). Have DHCP enabled to allocate some set of addresses for Network B in the 192.168.0.xxx range.
Set the WAN access to whatever is necessary to establish your connection to your ISP.
This will allow machines on both Network A and Network B to access the internet.
IF you want machines on Network A to be treated from a security perspective as local to the SME Server, specify a Local Network on the SME box with the WAN address of Router A (192.168.0.201) as the router address and the Network A address range (192.168.1.x) specified as the network to add.
Hope this helps.
John