You need to buy an alarm buddy, dont try to piece shit together to get the same effect as an alarm.
An alarm is the only thing that will do what you are asking, you need to be able to arm and disarm the system, simply wiring up sirens will not allow this.
Get a simple alarm, it will probably cost you something in the neighborhood of $100-$150 if you dont know how to install it yourself. Then just get extra sirens. They sell sirens with back-up batteries built into them for ~$30 on
eBay, I had one and it worked great in my Buick. This makes it so that they can NOT turn off the sirens, regardless of how many wires are cut. Or if you are doing several sirens, you might look into a small motorcycle battery, hidden, as a backup.