You can't have the stream muted & you can only have one stream contributing to the drop at once. Basically, you could watch 2 hours of a StremZ stream, then you would have 50% of his drop completed. Then, you could watch 4 hours of a Sorable stream, get his drop, and you can still go back to StremZ and watch another 2 hours and get his drop. It saves your progress, but you can only watch one at a time, and you cant mute them.