If the alt isn't keeping up, adding batteries is going to make things worse. Once a battery is below a full charge, which it will be as soon as the current demand surpasses the supply from the alt, it is now a load on the alt until it is fully charged once more. The ammount of a load that it presents is dictated by the internal resistance of the battery and the degree to which it is discharged. If you add more batteries to this you have reduced the internal resistance seen by the alt and increased the current demand from the batteries. The batteries will provide a bit of a buffer, but having two batteries won't double the time that it takes to drag the voltage down that low and once it gets there it will be more than twice as hard for the alt to dig back out.
A daily driver system is not the same as a SPL comp system. The daily driver is expected to run continuously and needs a constant supply of current whereas the comp system is only going to run for a minute at full tilt and then have a chance to recharge. To that end, a daily driver system needs a big alternator to supply the system and the car with all the current that it needs to run at whatever volume the user intends to regularly play it. A huge number of batteries does not do the daily driver a lot of good, if the alt can't supply the power needed at the RPM that thte vehicle is typically operated, then it needs to be replaced or tweaked (overdrive pulley) so that it does. Adding batteries is a very temporary band-aid but in no way fixes the problem. For the SPL system, on the other hand, The alt really has no chance of keeping up with the current demand of the system running at full tilt so huge banks of batteries are needed to make sure that there is enough reserve power to provide the needed current without allowing the voltage to dip much.
In this situation, I still have to think that there is a bad connection somewhere in the power supply chain. Have you had the battery checked again to confirm that it isn't bad and that it will hold a charge?
