How do you interconnect two networks in the same geographical area?

It all depends on this geographical area. You could use two wireless routers. Given the distance isn't too far. 2 linksys routers with ddwrt firmware, put one both networks and that will be a good job.

If it is far, you could be looking at running a wire from one network to the next.

If neither of these are feasible you could look into VPN's.

Have you got servers...??