Multiple batteries

I have a 1995 Acura Integra with a JVC KD-S990 head unit, Pioneer 6 1/2 components running of a Coustic 161SE 2 chanel amp, and a Coustic CF124 sub running off a Coustic 401SE 2 chanel amp (bridged, 400 watts RMS @ 4 ohms).  As may people probably know in this forum, honda products have nutoriously small bateries and I killed two with my old system that only drew about half as much current.  I want to set up a 2 battery system to give have more amps available at all times.  In talking with a couple aqaintances I wal lead to beliueve that unless an isolation circuits is used (which I have no idea how to set up) that the batteries must be wired in parallel (which makes sense) and that they must be identical batteries (?).  If this is so, why?  I know that many of you will say just get a cap, but I will be adding a second sub and changing over to an amp that pots out 800 watts RMS in a few months so I will need the higher voltage.  The expert advice of more experienced members would be greatly appreciated.

The only thing adding a 2nd battery will do for you is allow you to listen to your radio with the cars engine off for a longer amount of time, or allow you to drain your accessories battery and still start the car depending on the way you set it up. . What you will need with or with out a 2nd battery is a high out put alternator. The alternator will give you the current you need, not adding a 2nd battery. If you do add the 2nd battery it will put more of a strain on your current underpowered charging system, since it now has to charge 2 batteries plus supply power to your cars electrical system and any stereo equipment you have. By putting in a high out put alternator it will be able to supply enough current for your car, as well as your aftermarket stereo system.

Any recomendations on a good source for high output alternators?

If you tell me the total RMS of you final setup, I can tell you  approximately how many amps your new high out put alternator should be

If you park in front of a wall at night with your headlights on, and race your engine do the lights still dim? or dose it only happen at idle?

I would have your alternator checked to make sure you’re getting the correct ampage. Also what is the size of your ground cable?