I am not sure why as a consequence the smart access memory would create a second logical GPU. Very odd. Neat in your case but seems like a strange consequence.
It's definitely a bug. I disabled 4G Decoding, BAR in the BIOS, rebooted, DDU'd the drivers, reinstalled drivers, tested without Smart Access Memory, then tested again with it back on and now back to 1 GPU still.
Looking to see if something else I was doing caused it to pop two instances of the miner. The limited time I was running, it was in fact getting 80mh at the pool and had both compute 0 and compute 1 under load in the GPU monitor; now I just get compute 0 with the single thread.
Bottom line, bug or not, there is headroom here for dev to enable that second thread officially.