Adding a battery to the rear of the car will almost always be an improvement. However your high amp cuting our sounds like it could be "clip" related. If both amps power come from the same source and only one shuts off I would look other then power problems. A quick check with a voltmeter would answer both questions:
(1. Do you need a second battery and 2. Is the second amp cutting out due to low voltage)
It could be as simple as a loose ground to the high amp.
95 Contour SE '01 3L 
