I found several pieces of the obviously the same brachiopod species in one site (so they are from the same colony, maybe even the same family) in Yuangquan of Shanxi province, China( known for CP fossils)
the ventral shells seem to be double layered, with clay in bewteen now . This is true for all three piences, but more discernable in the last two ( as shown with circles). In the first piece the outer layer
remains are stripped to reveal a complete outlook.
I knew nothing about brachiopodes. But from the literatures I could get hold of, there is no description of double shell.