I don’t have my shrek anymore to check, but my recollection is there are four “balls” in that mechanism, two Newton balls on posts and two captive. The two captive balls were separated from each other by a post under the donkey pinball play field (should be able to see in the manual). By default every hit to the lower captive ball gave a pinball letter.
I do recall reading something about a more difficult setup where the post separating the two captive balls was removed and there would only be one captive ball. Then you’d need to alternate shots on the two. Seems like something that might be done to a papa machine. To get back to stock you’d need to remove the donkey pinball upper play field and replace that post.