I have 2 nic's in my machine with FC6 loaded.......

This particular problem has nothing to do with FC6. Any distribution of GNU/Linux will show the same result, I guess.

iam assinging two same range ip's i.e 10.0.10.1 & 10.0.10.2 with all the subnet mask

Is there any particular reason for allocating the IPs of same network to these interfaces? Why not assign IPs of two different network numbers?

and gateway address being the same for each of the interfaces......

The entry of default gateway in the routing table has a special meaning to it. It is to be used by the kernel when the destination address of the packet being pushed out is not reachable by using the other entries in the routing table. So it is not interface specific, but system specific. If there is an entry for the default gateway, then that address must be reachable via one of the other entries in the routing table. And every entry in the routing table has an interface attached to it.

i am taking the lan cable and inserting to the first interface and pinging the gateway, its pinging and its ok.

but when i take the same cable and insert into the other NIC it is not pinging the gateway.......

It won't; until the kernel knows that you have removed the cable and decides to use the other interface to move the packets to the gateway. It doesn't happen in the kernel automagically.

The problem here is pertaining to possibility of reaching same destination via two interfaces. That's why I asked in the beginning, why you want to create that possibility by assigning same network number to two interfaces. A packet goes out of the system via one interface only, even if the other is configured to carry it. In this particular case, the kernel is pushing the packet to the gateway via first interface. When you remove this interface from the switch/hub and connects the cable to second interface, kernel doesn't care. It uses the first interface only. You need to bring down the first interface using command "ip" or "ifconfig". Then only the second interface will be used to reach the gateway.

Hope that clears the matter.

Raghu