Depends, what size chickens, what size ducks? as well what are the dynamics of these flocks... roos? drakes? I keep mine separated, for one ducks are waterfowl, so that means they mess in the water and mess it up, chickens are not fans of this process at all.
As well there is a risk with drakes mating chickens which worse case can result in death of the hen, some are okay but that does have to be watched for.
Mine are "together".
They free range during the day together. 4 Indian runner girls and 1 welsh harlequin girl. The chickens are 2 BO girls, 1 Barred Rock girl, 1 polish girl and a polish roo.
The ducks don't tolerate the polish - they are low birds on the raking here. So any attempts by the roo to mate with a duck will not go well for him. I have seen him mating the polish and one of the BO girls.