Adding another siren will make it louder, also where is the other siren mounted and where is it pointed. Cutting the black loop will only make the arma nd disarm chirps louder, not the actual full alarm. As for them playing the same sounds, my experience with them is that not all the sirens are Identical and can eventually get out of sync if they are going off for an exteded period of time, but thats actually not a bad thing as it makes your car sound unique so you know for sure if its yours. I do recomend wiring in a relay if you are going to be running more than one siren as the output on the viper is only ment to run one siren. The sensitivity is adjustable on that unit. If there are a bunch of bars at the bottom of the remote than its adjustable by using the remote, if not then there is a dial under the dash either on the brain or on an outboard shock sensor depending on what its equiped with. The Proximity sensor means that instead of the alarm going off when someone hits your car, it starts warning them when they get near the car. Its a great sensor WHEN SET PROPERLY but when they arent they are annoying as hell.