ID not represent physical GPUs but GPU threads 1GPU two threads
In amd.txt for my 560s there is no "threads" option mentioned. What's your config for 550s?
look at video, there you can see config syntax for XMR-STAK and GPU threads
I looked, there is nothing unusual. I've got the same options. No two threads.
Have a look on the video at 57 seconds it's there.
This is another way to do the same thing, mod it for your own preferences.
"gpu_threads_conf" : [
{ "index" : 0, "intensity" : 800, "worksize" : 8, "affine_to_cpu" : false, "strided_index" : true },
{ "index" : 0, "intensity" : 800, "worksize" : 8, "affine_to_cpu" : false, "strided_index" : true },
],