Feasibility as a constraint on 'ought all-things-considered', but not on 'ought as a matter of justice'?

It is natural and relatively common to suppose that feasibility is a constraint on what we ought to do all-things-considered but not a constraint on what we ought to do as a matter of justice. I show that the combination of these claims entails an implausible picture of the relation between feasibility and desirability given an attractive understanding of the relation between what we ought to do as a matter of justice and what we ought to do all-things-considered.