If the amp is mono there is nothing to bridge. The extreme 500 watt mono amp is most likely the max watts. The two 12 inch 250 rms extreme subs, if they are single 4ohms each you can wire them at 2 or 8 ohms. If the extreme 500 watt mono amp is 2 ohms stable it will work. You might think about just hooking up one sub to the amp.
I have a 2002 Subaru WRX and hooked up 2 10inch rockford fosgate subwoofers in the trunk with a 2400watt amp. Now i was to put 6 by 9's in there so i can here the music behind the car too. Do i need to adjust the gain? If so how do i adjust it? Is there a special way of hooking the 6 by 9's to the system?