Occlude one polygon with another... need some ideas!

I'm trying to achieve an effect in TGB that I have not been able to figure out thus far. Picture two polys on the screen. One is to represent a light, meaning it brightens the scene within it. The other's job is to occlude the light poly, but not the rest of the scene. I would like to acheive this weith blending modes, but I was not able to find a combination of blending modes that would achieve this. I need a way to make one poly occlude the other, and not affect or show up in the rest of the scene. Does anyone have any thoughts on how to accomplish this?