You don't actually. But there are things you can do to help them like each other if it is not love at first sight. That happens, too, sometimes.
Set them up side by side - divide the living space so they can see each other but not touch each other at first. See how they react. They may fuss at each other. Feed them treats at the same time. When the fussing settles down, let them out in the same area and watch. A little pushing, shoving, and poking is normal. Even a little loud fussing. Again, they may just be best of friends right away. If not, put them back in their respective shelters, and try again later.
It took about a day for Michele and Romy to fit into the Runner flock. Bean, Hazel and Carmella seemed to fit in pretty well at first after a few days, but then we had X-rated middle school kinds of problems. Romy liked Bean. But when Bean started mating Romy, three or four of the Runner girls got all upset and jumped on Bean, who was on Romy, and poor Romy did not like that.
So from then on, Romy ran from Bean. Which really challenged Bean . . . so he ran after Romy. Well, it was a mess. So I split the flocks.
Fast forward. This winter, I tried again. Peace and harmony!!!! So now we have one flock again and I pray it continues thus.
Long story to illustrate two points. First that it is up to the ducks to get along. Second, we can very often come up with a plan that will help them get along.
