If it is a really critical system, the proper way would be a T1 circuit between the two buildings. The circuit would only go from one building to the local tele

If it is a really critical system, the proper way would be a T1 circuit between the two buildings. The circuit would only go from one building to the local telephone office, back to the other office. They tend to be pretty reliable, but expensive. You could then run a lan over this.