12V Charges at 14, 14 Charges at 16, 16 charges at 18. Are you talking straight up swap of 12v batts to 14, and charging at 16? Its been done, some cars can handle, some i wouldn't even attempt..
All comes down to the vehicle.
Hell, i think dbDon, is doing 18v daily? Or it maybe 16? Not sure on that one.
or, Add 2nd alt, do a complete different charging system, stock 12v, then 14v for the sub amps. Details man! details!!