I haven't been able to find any documentation that discusses the behavior of the volume wrangle when it is run on multiple volume primitives.
It appears that, if all the bound volumes have the same voxel size, the wrangle finds the bounds encompassing all of the voxels and runs the snippet once per voxel. If the voxel sizes are different, the wrangle iterates through each volume individually and runs the snippet per voxel per volume primitive. This can produce very different results if the snippet reads and writes from both volumes.
Can anyone confirm or correct this understanding?