You can start two miner instances with -di 012345 and -di 6789ab to confirm this idea.
As soon as OpenCL works with 12 GPUs, system starts to work slowly with sockets accepting.
I did some testing with your suggestion. When running up to 11 GPU's using the -di parameter, ETHman, (which is running on another rig) doesn't lose it's connection. As soon as I start a second miner instance for the other two GPU's, the first instance losses it's connection to ETHman and it only reports for the second instance with the fewer number of GPU's. Running 12+ GPU's in a single miner instance also causes intermittent ETHman disconnects. Is a core i3 CPU not enough?
As i mentioned ETHman reporting on the rig was working fine until about 2 months ago, so maybe the Windows v1709 FCU or new drivers changed something.