Unfortunately there is currently no offcial way of programmatically querying the HW for GPU load. However, please take a look at Intel Graphics Performance Analyzer (GPA) 4.1,http://software.intel.com/en-us/articles/intel-gpa/ This tool is free and provides an method for visually observing the GPU load level, via the "Media Performance" feature.
Regarding multiple encoding sessions. Yes, you can start as many encoding (or decoding) sessions as you want. Depending on your workloads, the individual workload performance will vary depending on overall GPU load.
Now, after my application is totally optimized, I have to say that I'm very impressed by the Intel Media SDK and encoding particularly. All together, the whole application with encoding process takes only about 7% CPU to encode three camera's streams in real time(1: 640x480 mono,300 fps; 2: 1344x1024 color,20.42 fps; 3: 1920x1088 color (bayer),15 fps)and save to the hard drive.