In this webpage from Nvidia, the author(s) seems to imply that you could create a command buffer for each eye on separate threads. However, I don't see the benefit to this over instanced stereo rendering since you must also handle synchronization with multiple command buffers.
Possible Synchronization Issues
For synchronization (feel free to correct me if I'm wrong), I was thinking about when using a framebuffer for each eye, kind of the "naive" way of rendering for VR. I suppose though you could just render to one framebuffer like with stereo instanced rendering. Regardless, the issue with synchronization that I'm concerned about would be if one eye finishes considerably faster than the other eye. It seems that with stereo instanced rendering, this would be less likely to happen since the workload is more divisible. The other issue I see where synchronization could cause a performance hit is that you must wait for both threads to complete sending their respective command buffer before sending it again. Arguably, this might not be a huge concern, but it seems unnecessary nonetheless.
Spatial Locality Issues
It also seems that since there isn't a way to guarantee when an object is going through the pipeline that you may get additional cache misses on the GPU compared to instancing here.
Is there any advantage to using multiple command buffers for stereo rendering compared to instanced stereo rendering?