How can I combine constant and switched power circuits to power a security light
You might be able to accomplish this by replacing the existing front light switch with a three way (SPDT) switch and running a new 3-wire plus ground cable from that switch to the back lights.  

It's basically a three way switch with a two way switch in series on one leg, where the motion detector is the two way switch.  

This would only work if the motion detector in the back is OK with this type of connection.  You'd have to consult with the manufacturer of the motion detector about that.  It's never come up for me so I have no idea if that runs afoul of the listing of those devices.  

At the back light, splice the black on the light, the red from the new cable, and the red from the motion detector.  Splice the black from the new cable to the black from the motion detector.  Splice all three whites.  Ground wires as usual.  

In the switch box, connect the black going to the front lights and the red from the new cable to the "up" position traveler terminal on the SPDT switch with a red pigtail.  Connect the "down" position traveler terminal on the SPDT switch to the black wire from the new cable.  Connect the common terminal on the SPDT switch to the black wire feeding the switch box.  Splice all three whites.  Ground wires as usual.    

When the three way switch is on, there will be 120V on the motion detector's red wire and 0V on the black.  That is not how it is usually wired, and may not be OK for the device, that's why I mention checking with the manufacturer above.  

EDIT:  

I gave this some more thought because I think it's an interesting problem / exercise.  I thought of another alternative that may or may not be worth the trouble.  

If you can't find a motion detector that's manufacturer approved / listed for parallel wiring, you could sidestep that by using a DPDT switch rather than a SPDT switch.  The second pole would be to disconnect the motion detector from the light and the parallel power when the front lights are turned on.  You'd have to run an additional 14/2 or 12/2 from the front switch to the light.  You probably see where I'm going with this but I'd have to make a sketch to explain it any better.  

That may be more trouble that you'd want to bother with.  Complicated is never good.  You may have to install a bigger switch box up front and / or a bigger box / second box in the back.