The magnetic driveway probes operate as being part of a complete circuit/loop which is being monitored. So a probe with 50 feet of wire for example is in a circuit which consists of 50 feet of wire, plus the probe. When a fluctuation across that circuit (caused by nearby moving metal) is detected it triggers the alarm event. If you are using relatively short runs of wire and can turn the sensitivity on your system down than using two probes on the same circuit may be ok, but generally we don’t recommend it as it can cause false alarms (the circuit becomes larger, and more sensitive).
If you’re looking at a wireless system though such as the Dakota Alert DCPA-2500 you can simply add another complete driveway sensor with transmitter to the system which will work in conjunction with your original sensor and chime allowing you to monitor multiple locations at once!