It has been shown that ex post efficiency can be attained if initially both parties own 50% of the good to be traded.
ɪt hˈæzbiːn ʃˈoʊn ðæt ˈɛks pˈoʊst ɪfˈɪʃənsi kæn biː ɐtˈeɪnd ɪf ɪnˈɪʃəli bˈoʊθ pˈɑːɹɾiz ˈoʊn fˈɪfti pɚsˈɛnt ʌvðə ɡˈʊd təbi tɹˈeɪdᵻd.