I used to run 2 main sirens on one of my old cars witout any problems. As for how much current the brain can handle, i'm not sure on exact numbers, but running 2 main big sirens will be fine. IMO, dont go over that unless you use a relay to power up additional sirens. YOu can put that PG anywhere you want really. Inside vents is an excellent location, as its hard to get at them, and also is hard to find them.

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)