Why not let Miss Fortune's Q apply Sheen effect on both targets?
It would be cool to let this exception of Sheen happen since Trinity Force is decent on her and its weird to have the Sheen effect apply only on the first target when you are actually trying to do the most amount of damage to the second target. At least let Sheen apply only on the second target and not on the first.
or you could unlock the secret powers of all Miss Fortune mains and buy 2