2 8D's should work
You can wish in one hand and $hit in another. Which one do think gets filled first?
In all honestly it sounds like you are going to be asking a lot of your alternator/battery bank. So amps=watts/voltage. 2100/12=175 amps. No your amps aren't going to actually be pushing that many watts the entire time so the amp draw could be a lot less but I have no idea how much. 50% less? 25% less? 10% less? I think the type of music you listen too, how well your stereo is tuned, how much head room you have, what ohm load your amps are running at all a variables I think.
Now lets think about the alternator for a second. How are you going to separate the banks? Perko, ACR, a combo, etc??? If you use an ACR like a sure power you may have problems ever getting the switch to combine long enough to even charge your stereo bank assuming you plan on sitting with the boat off jamming like most seem to do a lot of. If you use a perko style switch you will have to leave the switch on both or the stereo bank. If you do this for long you are asking your alternator to go full blast for too long(if you actually want to charge up the depleted stereo bank) which is going to seriously shorten the life of the alternator. Also, if you think you are just going to start the boat and let idle for a30 minutes and it is going to fully charge the batteries you are expecting too much and abusing the alternator. Remember the rated voltage of an alternator is max and not at idle. Your 70-90 amp alternator might only be putting out 40 amps at idle. Not near enough to keep up with your stereo let alone charge the batteries.
When cmaping for an entire weekend I think the best bet is to have an onboard or portale 20 amp+ charger and using a portable honda generator for a few hours in the evening when not using the boat. Then just leave the stereo bank separate from the starting bank all weekend.
Another option is you could buy a 150 amp Balmar alternator from me for ~$750. The problem there is you will have to upgrade your cables as well and you need to have at least a serpantine pulley. A single V belt won't work.