Excellent and thanks for doing the tests.
This is what I expected. One can think that the more GPU´s the better. However, what happens is that the CPU must administrate more and more. First which of the GPU´s to send data to and second administrate the data communication between the CPU and GPU and third combining the data. This administration takes a lot of CPU power and therefore it increased to around 90%. In this case using only one GPU is much more efficient. Most likely it is necessary to have a more powerful CPU to handle multiple GPU´s.
There is also a difference between the graphics cards as they use different technologies. The older Quadra 2000 use a technology that focus on decoding of h264/265 data streams. The newer graphics card Grid K1 use a technology that focus on encoding of h264/265 data streams to produce higher quality video content on monitors.
The last question why according to the Task Manager the Encoder is not in use. Is NCS in motion detection or is it set to continuous record a video stream? There are two things involved here. If set to Motion detection NCS will use so little of the GPU encoding power that it do not show. Second is that the Task Manager has problems to show correct data depending on internal or external GPU. The Decoding seems always correct, but encoding is not always correct. If you have another program that shows the Nvidia GPU load it might be better.