Stand alone charging, 12v 14v 16v on 8k+

[Pasta Prime]

I'm wanting to install a second alt in the ac spot on my 94 civic hatch, There's plenty of room it will just take some fabrication and magic to get it perfect. So my thought was to get an elite series 370 or something similar and use it separately from the cars charging system.

Now the goal here is two 18s on two 3500/3800 but until i get the wall shell built to see how much space I'll actually have subs aren't firmly decided yet.

So here's the question; if I were to use 14v or 16v batteries can I get away with less of them than using a 12v system? Lithium aside of course. I want to be able to maximize space for the wall but still have enough electrical for two 3800/3500 amps plus a 1100.4 or similar.

Also, how many 12v 14v or 16v batteries would I need to keep up with 8k+? Plus, will the single 370 alt be enough?

Is there a big difference in maintaining the higher voltage batteries than the 12v other than the charger?

Has there been anyone do two alts on a civic? I did a lot of googling last night and only seen one video where a guy was working on installing one where I want to. The only issue might be is that is to close to the header so there might be a heat issue.

Is the mechman elite series just a large case gm alt? the ad222 case or whatever it is?