Two groups of ducklings = turf wars?

You need to keep the separated until they are larger.
My 10 2+week old ducklings will attack our 1+ week old goslings if left alone. So we keep them separated until their sizes even out more.

 

I've found ducks to be much easier to introduce to each other than chickens. I had 3 that were 5 weeks old, and 2 days ago I added my 3, 3 week olds from the brooder. The big white Pekin was a little rude, but the other 2 took them instantly. They grouped together and talked about it. The Pekin and Blue Swedish I think are male, and they put their heads over the backs of the others a couple of times in some sort of dominance thing.

But on day 3, today, they're acting like they've been together this whole time. The 3 weeks olds were big enough to handle a little pushing around though. With yours at 7 and 1-2 weeks, that's too much of a size and age difference. You can expose them to each other so long as you supervise at this point. But it will be another 3-4 weeks before you can really let them sort it out.

I'll have to do it again in a couple weeks with my remaining 3 in the brooder. One is just shy of 2 weeks old, and the other two are as old as the others, but much smaller, so I left them with the youngest.

It's cute though, there isn't really a defined leader of the outside 6, and my first hatched baby will come to me and it's two friends will follow it even though they don't care for me at all since I didn't hatch them. The oldest 3, then follow them over, since they can't stand it if half the flock leaves. So there is 1 that is making the other 5, who were all gifted to me, come over and visit.

Ducks are so goofy!