1. The physics of sound don't follow conventional thinking...... I'll put it like this: a Marshall Full Stack guitar amp is only 6db louder than a Half Stack, yet you never see Jimi Hendrix, Jimmy Page, or David Gilmour use half of a Marshall Stack amp! (the full stack is considered just for show). Hence, two sirens aren't much louder than one. The only attribute of having two of them, you can mount one in the trunk, and have the sound in multiple directions.

2. You can install a 130db high frequency siren under the dash..... that way if someone acutally makes it into the car and gets under the dash to find all the wiring, they have this pizzo-electric buzzing painfully in their ear.

3. Refer to answer to #1.

4. That's the right spot.... hidden, difficult to access, and hide the wiring (loom and black tape everything, so you can't tell what is what!)