Car could be on one voltage (stock alt) and have a secondary alt with higher voltage just feeding the amp... I did it for years on the CB... 12-14v for the truck and 16-21v for the amps I was running... As long as you keep your second charging system isolated you are good...
Whole car is on 16.1v. Takes it just fine lolZero issues
I know about the dual alt set up but I knew he only had 1 alt And in this case refer to my avatar SD