I had similar problems too when I was a begineer. I applied a lot of pressure on my left hand fingers when my both hands were engaged. I guess this happens because our brain is not used to that kind of multi-tasking. It needs training. The way I was able to fix that was by only practicing my fretting left hand enough prior to putting both hands together. I am not sure if it was only me, but things were completely different when only putting my left hand at work.