Which works fine if you are doing a hands off approach and letting the ducks hatch/raise.
For those that have their ducks as pets or even food/eggs but are brooding with no mother duck, existing adult ducks often want those little buggers gone, so we have to do a more introduce and see what...
You have a good start - make it so they can check each other out, but not get to each other.
Then supervised time together.
Then I do some time alone together.
Then I put them in the same house at night - I divided at first, hope I can avoid that this time.
Time and patience