California Vehicle Code:
http://www.leginfo.ca.gov/cgi-bin/calawquery?codesection=veh&codebody=&hits=2024003. No vehicle shall be equipped with any lamp or illuminating
device not required or permitted in this code
24600. During darkness every motor vehicle which is not in
combination with any other vehicle and every vehicle at the end of a
combination of vehicles shall be equipped with lighted taillamps
mounted on the rear as follows:
(a) Every vehicle shall be equipped with one or more taillamps.
(e) Taillamps shall be red in color and shall be plainly visible
from all distances within 500 feet to the rear except that taillamps
on vehicles manufactured after January 1, 1969, shall be plainly
visible from all distances within 1,000 feet to the rear.
(f) Taillamps on vehicles manufactured on or after January 1,
1969, shall be mounted not lower than 15 inches nor higher than 72
inches
I would say it's permissable, you are allowed more than one, as long as it is red and visible at 1000 feet.
Not to mention that any ticket would be an equipment violation and thus a fix-it (no charge!) ticket.
I would
hope that any LEO would give you an attaboy for trying to be more visible, even if they declared it not legal.