This was reported to R&D (#95588) for further investigation. There is not a known workaround for the error at this point.
I have tested a different but similar setup and I was able to verify that a multiple device task (i.e. AI channel expansion) does work with DAQmx 8.5, LabVIEW RT 8.2.1 and a PXI-4472 and also with the latest versions of DAQmx and LabVIEW RT. Here are the details of the test setup. At the time of the test I was unable to find a PXI-8106 RT controller.
PXI-8186 RT Controller (RT OS)
2x PXI-4472 DSA devices (starting in Slot 2)
LabVIEW 8.2.1 and LabVIEW RT 8.2.1