I am not sure that I quite understand your question. You say that there are two locations (two different cities ?) and a leased line. Is the leased line already in place or is it to be installed?
I am not clear whether your comment about the sites having different IP & subnet applies to their LAN (in which case it does not matter and in fact it would be a problem if they did not) or if it applies to the leased line. Can you clarify?
In the normal case you install a leased line between the sites, terminate the leased line on serial interfaces, assign an IP subnet for the leased line, assign host addresses from that subnet to the serial interfaces of both routers, establish routing logic over the leased line so that both sites know how to reach each other. That routing logic could be based on static routes at each site or could be based on running a dynamic routing protocol over the leased line.
HTH
Rick
HTH
Rick
