Planar objects with opacity maps matteing out the fume smoke

[Suspensa]

Yeah I also thought I might be missing something, but I have yet to figure out what that something is. I've tried different mapping techniques, rendering settings, etc, but haven't been able to solve it. I can just composite the fume smoke in the meantime, but at this point I'm just curious to see what the solution is.

Thanks for the help,

Spencer