I have one circuit that controls outside lights where the light poles have electric photocells and porch type lights have their own built in electric eyes...
No problems so far...
However, I just replaced a post electric photcell and it has a NOTE that says DO NOT USE ON CIRCUIT THAT HAS ANOTHER ALECTRIC EYE ON IT????
What is up with that... a person may not even know if a circuit has another photocell on it... (dumb I know but true... ) but the...
QUESTION IS: WHY IS THIS EVEN A CONCERN??

Not sure if there is a code on that, if their was, it may be because a circuit you test has no power, then a cloud goes over or darkness falls, that wire is now energized, most would not think of being on 2 photocells.
If they don't electrically work in series, it may be to the low load of the photocell?

You could wire 2 in Parallel, so if Either sees dark, the light will go on.

Everything seems to be working fine.
The question is WHY or WHAT happens IF you have more than one electric photo cell on one circuit??

I have seen this WARNING before but do not understand the risk and I have had more than one on the same ciruit for years??

My latest phot cell is a WESTEK Outdoor/Exterior Post Eye Light Control... on that back of the package it says:
NOTE: Do not use this control with another control on the same circuit!

I do because it has been this way since I bought the house and want to know WHY and WHAT ARE THE RISKS??

If it works, it should be fine unless otherwise noted.
Personally I don't see a problem, unless the lower resistance/voltage at each photocell has been known to cause a fire or something?

The only possible issue is NEC 110.3(B) Which requires products and
Devices to be used and installed in accordance with the manufacturers
Instructions...