Get two batteries it's not gonna strain your alternator.. Running big amps without a cap or a extra battery will slowly ruin ur alternator. However its a good idea to run 2 batteries that are the same. I run 2 hc600s with no problems. They've been in for about 3yrs
For those who are going for sounds systems with a bang, IMO caps are utterly useless unless your getting high ends $300+, rather get a second battery or performance alternator. To answer OPs question, no you don't need an additional alternator or performance alternator to run two batteries efficiently.
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