Well you need to know what kind of trigger your alarm is, or what polarity the alarm is putting out to the siren. Most alarm systems use a positive wire that goes to the siren, and then just ground the neg wire of the siren, but some alarm systems use a neg wire that comes from the alarm. To be safe when using 2 sirens with an alarm, you should use a relay. Get any 12volt SPDT relay. Assuming you have a positive signal coming from the alarm system: Pin #85 on relay-hook up to the positive 12v siren signal from the alarm. Pin# 86-hook this to ground. Pin# 87-hook up to battery or any high current 12 volt constant source. Pin# 30-hook this up to the pos lead of the siren, and just ground the siren's neg to ground. hope this helps, any more q's, just speak up