In this particular case, I think the reader would assume 2 bodies with any form (unless Sam and Michael were Siamese twins). Sam and Michael's toy-- they share the single toy. Sam's and Michael's toy-- they each have their own single toy, so there are 2 toys.
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AnonymousSam's and Michael's bodies were thrown into the pond. or should it be Sam and Michael's bodies?What if 'bodies' was replaced by 'toy' and the toy belonged to both of them. How would the possessive form look?Thank you, teachers!The possessive form goes to both -- as you have done -- if you're referring to two separate bodies, those of Sam and of Micha