When I say "John's sugar and Tom's sugar are different", I know it must be plural because there are two items in the subjective.
However, in "John's and Tom's sugar", there's only one item, so I should use "is"? But it's referring to two things, and telling people they are different, so it seems that "are" is more logical, right?
Or should I say "John's and Tom's sugars are different"? I know sugar is uncountable, but sometimes uncountable nouns also have plural forms, like "We'd like two coffees". But I'm not sure whether that's correct in formal writing.