I'm afraid Lux is not smart enough to know that the light is actually behind that layer. You can use two instances of Lux and have one render the lights in front and one that does the lights behind. You'd need to name the lights "Front"... and "Back"... and use the General>Name starts with "Front" and "Back" on the two instances. Of course if one light moves from the back to the front this approach won't work. In that case the lights need to be switched on or off at time of obscuration or appearance.
Currently, there is no way for an AE plug-in to know about all other layers in the comp, it's just not in the API.