What is the common workflow for using multiple VDB caches with density+heat+temperature in one render scene?

[beyczy]

Hello!

In our company we are using Maya 2017 + Arnold 4. We have a scene with a pile of cash catching on fire. The fire consists of two VDB caches. One with the upper flames, which emits smoke as well (scattering driven by density channel, emission driven by heat channel), and one with the lower flames spreading downwards without smoke. When I use only the flame component of these VDB-s, the two flames appears in the render as it has to be. If I hide the lower fire VDB, and use the upper fire with smoke, it renders fine as well. However if I enable both fires AND the smoke component of the upper fire (raising the scattering anything above 0.0), the bounding box of the upper fire obscures the lower fire completely. Am I missing some settings? I don't think it's impossible in Arnold to render multiple VDB-s with scattering and emission without obscuring each other. Thank you for the answers!

András