I am currently working on a migration project. We have two LANs running parrallal during the migration, one 192.168.1.0 and the new 10.1.1.0. Now I have a user (10.1.1.6) needs to access a server (192.168.1.7) on the other LAN. No other inter lan access is required. Now if I put a 2nd NIC on the file server and connect that NIC to the 10.1.1.0 LAN switch and assign 10.1.1.10 static IP , gw 10.1.1.1 on the 2nd NIC, will this is work? Our objective is to only provide one user (10.1.1.6) to the server and every thing else will remain unchanged and working. Thanks for your help.
If you do what you are suggesting then all 10.1.1.x addresses will be able to connect to the server on it's 10.1.1.10 address.
Also having 2 default-gateways on a server is not a good idea and in the scenario you describe you don't need it. So you could just address the 2nd NIC as 10.1.1.10 but do not put in a default-gateway of 10.1.1.1.
That way a host in 10.1.1.x can still communicate with the server and because they are on the same subnet the server can communicate back with the host.
However that doesn't solve you other problem ie. only allowing 10.1.1.6 and no other 10.1.1.x addresses.
Do you not have a router or L3 switch that connects to both LANs ? If so the easiest thing to do is use an acl to restrict traffic between the 2 LANs so only 10.1.1.6 can talk to the 192.168.1.7 address ie. you don't then need a 2nd NIC.
If you don't have a router you may be able to use a firewall on the server and control access to the 10.1.1.10 NIC address.