Not sure,
but I believe they are both male and trying to show each other who is boss. My male muscovy and my old rooster used to have at it every chance they ended up in the same run, which my Ducks would come and find where the chickens were every so often, I keep the duck down by my creek and my chickens closer to my house.
Lately I have been keeping the ducks penned up due to SIL problem, she is my neighbor and hates my ducks! Anyway, a few times I have let the ducks roam, they came to visit the chickens, all seem to get along, but the 2 new roos are just cockerls(SP)
Like I said given time to grow I would think that the roos would feel the have to protect their ladies from the male duck and would fight with him.
The muscovy is a pretty good fighter so I would try to keep them apart before one of them ends up hurt bad!
Brenda