i have installed two LAN cards in my PC. one is used for local networking and other is for Wide area networking( both have a different gateways). But when i need to browse in WAN for that i have to disable my LAN and vice versa. so i don't know the technical / logical reason behind that. so could you please give me reason why this happens.
The reason for this is that you have two gateways, your machine does not know where/how to route packets.
You really need to manually configure the routes (here is a guide to help you).
Alternatively, make sure that the subnet is configured correctly on the LAN and disable the gateway on that interface. This will allow you to communicate to all traffic on your local network, but anything outside will be forwarded to the WAN gateway.
Hope this helps!