You can do it your way , 2 separate panels , the 1 in the garage should at least have a sound-bomb wired in , (whats the point of an alarm if it doesnt make a din!) using the relay method , wire it into a spare zone on the house alarm ,programmed as Fire , then ensure the garage alarm has a shorter ring time than the house alarm.
Relay http://www.securitywarehouse.co.uk/...age=product_info&cPath=36_46&products_id=1575
Relay http://www.securitywarehouse.co.uk/...age=product_info&cPath=36_46&products_id=1575