Too late in the season for what? It looks like you're in Wisconsin, a hard winter makes birds huddle and bond when they wouldn't in the summer. You have two that like each other, I would get three (minimum) more at an age that they'll fully feather out by, say, october.
It allows the three+ to keep each other company and the winter (I'm in Michigan, I'm familiar) will allow the entire flock (the three+ plus the existing ones) to, if not bond at least commingle and be comfy with each other by spring. You may never end up with a cohesive flock but a winter will give them the best chance in my opinion. But I defer to Miss Lydia on this (and everything I say).
And if you can (and can have the right duck/drake ratio) try to have two drakes. I know from experience it's terrible for all involved to lose a drake. Ducks really want a drake around. If you lose one, you lose offspring but you also chance having a duck turn really mean trying to fill a drake's role. And a duck-turned-drake is meaner than a goose. They just try too hard.
I don't mean to contradict because you have many, many more times experience than me. And, by and large you're right. I love scovy drakes. But I had one that was just awful. Really terrible!
It would hump anything (chicken, young goose, runner, scovy, ANYTHING whether they wanted it or not). And still go after it when I pushed ("pushed") it away. It would rape a raspberry bush, thorns and all. He was terrible!
But out of five, he was the only one I called Genghis.
The other four were just great and exactly as you say.