Since Alembic doesn't offer stacked knobs, you're looking at a vol-vol-filter or vol-pan-filter setup. I don't think either would be right for a stereo config. You're going to need at least vol-filter times two for that.
If it's built like the B8S that I briefly played with, there's plenty of room in the control cavity for more than Hamer put in. You may need to drill another hole or two, but there might be space for what you want. Of course, someone may have built a stacked pot of a quality that Alembic would approve of, so I would ask that question before charging the drill batteries.
-bob