i seperate them so they can see each other but not hurt each other. after a hr or 2 of that i just let them in with the other and after a quick wrestle everything normaly calms down and with in a few days there best buddys
If they're young ducklings, or just ducklings still in their fuzzy stage, you can probably just stick them to together with no real problems. That's how it worked when I hatched out mine over the course of a week.
Just stuck 'em in with the others, and they all got along fine.