Two rear fog lights are legal in California.

But, I also agree it is a bad idea. The person behind you will think
your brakes are on. Then, when the brakes are really applied, the
person behind you may delay in applying their brakes (since they
have been fooled up to this point). Seems that increases the
chance of a collision (rather then reducing it).

I also find flipping on the single fog light causes
tail-gaters to back off (so it works just fine, as is).


California rules on use of rear fog lights from vehicle code
section 24602:

24602. (a) Any vehicle may be equipped with not more than two red
fog taillamps mounted on the rear which may be lighted, in addition
to the required taillamps, only when atmospheric conditions, such as
fog, rain, snow, smoke, or dust, reduce the daytime or nighttime
visibility of other vehicles to less than 500 feet.
(b) Such lamps shall be installed as follows:
(1) When two lamps are installed, one shall be mounted at the left
side and one at the right side at the same level and as close as
practical to the sides. When one lamp is installed, it shall be as
close as practical to the left side.
(2) The lamps shall be mounted not lower than 15 inches nor higher
than 60 inches.
(3) The edge of the lens of the lamp shall be no closer than four
inches from the edge of the lens of any stoplamp.
(4) The lamps shall be wired so they can be turned on only when
the headlamps are on and shall have a switch that allows them to be
turned off when the headlamps are on.
(5) A nonflashing amber pilot light which is lighted when the
lamps are turned on shall be mounted in a location readily visible to
the driver.