Try to use 2 threads/GPU.
"gpu_thread_num" : 12,
"gpu_threads_conf" : [
{ "index" : 0, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 0, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 1, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 1, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 2, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 2, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 3, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 3, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 4, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 4, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 5, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
{ "index" : 5, "intensity" : 512, "worksize" : 4, "affine_to_cpu" : false },
],