Hi James - Great Question.
Here are the options to control one of two fixtures wired to the same switch: Testing and Investigating - Voltage Tester is needed - The switch box in the house would need to be opened and tested to see if the power originates in the switch box where there is a hot and a neutral OR is there a pair of wires coming into the switch box where the White and Black wires are attached directly to the switch. If the later is true then the power originates at one of the fixture boxes, most likely the closest fixture to the house switch box location. After identifying the fixture box where the power originates AND if this is the location where the motion/camera is to be located AND if the second fixture is wired through this fixture box THEN you isolate this fixture from the switch leg, wire the switch leg through to the second light fixture and then attach your motion/camera to the originating Hot and Neutral in this fixture box being careful to leave the switch leg wiring in place to maintain the home switch control for the second fixture. Some Cost Involved, But a Quick Solution - Purchase an X-10 Control Module and Switch. These devices are fully addressable and work very well together. The wall switch could be a WS467 Wall Switch Module and the light fixture to be controlled apart from the motion/camera could be an Inductive In Line Module - XPFM. Look under the Section X10 Home Automation Accessories on our website for the Wall Switch Module. This new store module was recently installed and works really well complete with online help which may need to help you locate the Inductive In Line Module. This In Line Module is NOT the Plug-In type, it is the type with the wiring to be connected to your light fixture. Using these X-10 Devices would require the original switch to be removed, joining the original wiring which attached to the switch thereby making both fixtures hot all the time
The WS467 Switch will need to be connected to a Hot and a Neutral wire at the switch box. Then installing the XPFM Module inside the fixture box of the fixture to be controlled. The WS467 Switch and the XPFM Module will need to be addressed identically. Once these devices are installed and addressed the WS467 Switch will operate the outside light fixture and the motion/camera location will remain hot all the time. The X10 Devices really do a great job. I use them to control the lighting in our home and while we are away a stand-alone controller automatically controls all the lighting giving the appearance that the home is occupied. There are a lot of great applications X10 can be used for, your situation is one example. You may also want to look into the camera options which give remote control capability as well. Moderately Difficult: In the location of the proposed motion detect/security camera - Is there a outlet on the wall directly below this outside light? If there is you could extend the outlet wiring up to the location of the outside fixture box and cap off the original fixture wiring thereby isolating these two power sources from each other. This solution is basically a process of locating a circuit in the general location and extending it to the outside fixture box. More Difficult: If the power originates at the house switch box location the tedious process of installing an additional wire from this switch box out to the fixture box location for the motion/camera. Depending on the layout of the home, attic access etc this would be the last choice.
This should help you get your motion detector light and camera device working and maintain switched control of the outside light fixture.