Full Idea
In modal realism, 'accidentally coextensive' properties are not coextensive. They only appear so when we ignore their other-worldly instances. If we consider all instances, then it never can happen that two properties are coextensive but might have been.
Clarification
See Idea 15732 for 'accidentally coextensive'
Gist of Idea
Accidentally coextensive properties come apart when we include their possible instances
Source
David Lewis (On the Plurality of Worlds [1986], 1.5)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.51
A Reaction
It is not clear why this 'can never happen'. Maybe even God can't make a hearted creature without kidneys. Lewis is aware of this question.
Related Idea
Idea 15732 Properties don't seem to be sets, because different properties can have the same set [Lewis]