Well, more information related to the vertex mapping into the opacity channel.
If you overlay a material with displacement being masked trough a vertex map in the opacity channel it won't mask. But if you turn on the displacement channel for the underlying material it will work.
Curious fact, for the "base" material: If you set the displacement distance to zero, both material will inherit the distance from the "top" one.
I suppose that is not the desirable behavior, so please, would it be possible to address this issue?