I've finally achieved my long-term goal of double, side-by-side composting bays (triple, if you count the green Dalek). While one bay is left alone to mature, the other one can be filled up and tossed about. Visible and invisible composting critters can pass freely between the bays.
It's hard to express how much joy was involved in this whole enterprise.
On reflection, the bays would have been much more convenient divided the other way with the bays open at one end for ease of access. Achieving this will have to be a source of joy for another time.