Each individual machine will take the same time to reach a given sl wether it's coop or not. After the render is stopped the mxi's from each coop machine will be merged resulting in a higher quality image with an unknown sl. Make sense?
Rob, most likely what is happening is the MXI file and the image file that is being copied to the folder you specified once the merge process is complete is only the result from one of your computers not the merged result from both of them. The way to check it would be to go to the Next Limit folder on both machines and copy the cooperative.mxi file from each, then manually merge them together and see if the result is different from the original one you've got now. This is a bug but it doesn’t happen all of the time.