How to wire?

The black on the photo cell needs to be a constant or switched hot. And the red only connects to the 2 lanterns hots. White goes to neutral.
If this is done in romex, you may not have the extra wire.
With conduit, you should be able to push or pull a new wire in.

Strat
You have me really confused.

Mpbentinc;

I seriously doubt that you have two light connected in a series. Why would you do that?
I suspect that what you are saying that you have two light you want to control with a photo cell. That the wiring goes from the photo cell to the first light and then to the second. Sure they come one after the other in the circuit but that does not mean that they are connected in a series (electrically). They would be connected in parallel (electrically)

While you said photo cell, do you mean motion detector (with or without photo cell)?

If you mean two lights controlled by a motion detector, connect the black of the source to the black of the motion detector. Connect the white of the source to the white of the motion detector and to the white to the first light. Connect the red of the motion detector to the black going to the first light.
At the first light connect all whites and all blacks together. At the second light connect the white to the white of the light and the black to the black of the light.

HK,
I don't think the lights are electrically in series. I believe the wiring goes from one to the other.
I see no mention of Motion Detector, even though they would wire the same way.
I will try to say a different way. Tie all whites together, Black on the photocell connects to power, red on the photocell connects the light hot wire(usually black).
A problem with the photocell close to the light is the light MAY turn the Photocell/lights off and on repeating.

Sorry Strat, wasn't think about distance between photo cell and light. I was thinking you were saying he would need an additional wire.

Just old age.