How old are they? My runners started flirting with each other at about ten weeks of age. The nipping preceded grabbing each other by the back of the neck and playing "hop on top." Normal hormonal activities for maturing ducks.
My ducks don't seem to worry about gender as their breeding hormones get going. They just go with the feeling, so to speak. So it looks to me like your boys are beginning to reach sexual maturity. Aggressiveness is something that some drakes have, some drakes don't, so keep an eye on them. I wouldn't worry unless their play becomes rough. If there are no females around, I doubt they would have any real conflict.