I've had the opposite experience -- they got along as ducklings, then when everyone matured, they get along exactly the same. Seem to share the girls... We have loose definitions of family relationships.
That said, I had (have) a plan to separate them into different areas if ever needed.
I have two drakes and three hens who are two years old. The boys fought over who was boss when they were young. Now, they are fine with the two of them. However, they attack and rape the other guys girl(s) when their hormones are high. So, from April through July I have to keep the two groups separate or they are too hard on the girls. It is a pain. I would be better off with one or no drakes but I really like both of them.