I'm currently looking into a 2 processor solution with my 861.
How can you do this as Atmos only works with bitstream and not PCM.
Yes, the bitstream would go to the first processor (an Atmos capable pre\pro) which would do all the processing and churn out an analogue 5.1.4 Atmos layout ), i.e. standard 5.1 + 4 height channels. This processor would also control the system volume and feed a power amp for the 4 height channels. The 5.1 channels would go through my 861 via the analogue inputs to the DSP speakers (or whatever). So the 861 would solely act as pass-through device and would be set to a fixed high volume setting.
Not the simplest of set-ups (but you did ask). I'm going to wait a while to see how Atmos takes off but I think the above is doable and easy to automate.