Why couldnt duncan send Horace to falaise to challenge lassigny to a single combat duel?

We know Horace was better than lassigny and that lassigny was a tyrant that couldnt afford to reject a duel since he would Lose respect among his soldiers, its the same reason why deparnieux couldnt refuse a duel with halt. So why was it necessary for halt and maddie to go there in disguise when they could just go straight to falaise and lets Horace challenge lassigny?