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It is really common with the stereo guys and somewhat common with race cars. You use 2 8 volt batteries in series and an alternator setup to charge 16 volt batteries.
On race cars you do it for a stronger spark. In the audio it is done when you want lots of power out of amps that have unregulated power supplies.
Some guys have an Accuvolt in the system for electronics that cant take 16 volts.
I help crew a 615 CI BBC altererd drag car. It has 15-1 comp. and is injected on alcohol. When we switched from 12v to the 16v race battery it made all the difference in world and solved all our hard starting problems. It would crank noticeably quicker.We do not run a charging sys and it is charged between rounds with a 16v charger. Many of times I have wanted to put the 16v in my 7.3 in the winter but my partner go to Fl for the winter and everything is all locked up. I don't know if you can hook up 1 12v to 1 16v because the 12 would draw down the 16 and you would end up with maybe 2 14v. They do have a 16v battery on the market with 1 12v post so it comes with 1 neg and 2 pos posts both 12+16. I recentlly changed a 39 Ford from 6v to a 12v system. That old flathead fires right up now. So more volts give you quicker cranking for easier starting
The stereo might not handle the extra voltage. I would check the voltage range that it can operate safely. Hate to blow out a transistor and have a large repair bill.
I've found my amps, the warrantee covers 16v operation. They also warrantee them if you run them as low as .5ohm as long as you have two batteries or a high output alt.
Most Pioneer decks can handle a 16v system. I think Alpine can too.