Your headunit is cutting off because of low voltage, ive had it happen to me. Those amps draw 200amps of current each, thats a total of 800 amps. your alternator is 80 amps IIRC probably 50 amps at idle. The batteries once charged will power the amps for a minute or two until they are drained and need to be recharged which is what your alternator is for. you are going to need a lot bigger alt than what you have now to solve your problem.