Hey! Just a quick thought on this based on the experiences that I'm having with my small flock.
I'd never seen this information put this way or explained quite like this before which is why I want to chime in even though I don't have much experience. I think people when mixing flocks seem to think that the biggest concern is that the two flocks don't know each other; therefore, they are logically going to fight. I'm finding that isn't necessarily the problem. What you have to worry about
isn't the fact that they don't know each other. It's the fact that at 2 weeks of age versus 9 weeks of age, they are developmentally TOTALLY DIFFERENT. I have four ages of chicks in my flock and this is what I'm seeing:
The Barred Rocks are the oldest. At this stage in their development, they are learning how to fly and how to mock fight. They are standing face-to-face and bumping chests with each other. They take very thorough dust baths. They play rough but that's what they're learning right now ... that's where they are in their development. Just younger than those are the RIR's. They seem to be mastering the art of roosting on things. They seem very docile and aren't into practicing their flying or their fighting. They are very into making little nests with their bodies when I take them outside. They're trying to master the dust bath. The youngest I have are the Leghorns and they're into speed right now. Everything for them involves learning how fast they can do things. Fast eating. Fast running. Fast flying practice. They are also learning how to scratch to search for little bugs and things in the grass. My little bantam Sultan is only a week old and he seems like a short bus rider compared to everyone else ... but he's doing fine for where he's supposed to be at a week old.
You have to worry that the 9 week old is learning and experimenting with actions right now that your 2 week olds are nowhere CLOSE to learning and the two week olds need to come across that knowledge when it's natural for them ... not being forced into it because that's what the older chick wants to do or how the older chick wants to play. I'd keep them separated until they're a few months old and they've been able to learn all the things that juveniles should know. I worry that they are going to be "stunted" in their development because they're going to be getting bossed around by the older one who wants to practice the things that are starting to come naturally to him or her ... and it will happen at the expense of your two week old's not getting to do the things that they should be learning to do at this time in their lives.
Anyway I'm not sure if that makes sense. It's just that some people will maybe think "Well if the 9 week old is nice to them, I'd put them together" but I just think that they each need space and time to learn the things that are starting to come naturally to them without being hindered by the other age group.
But again, maybe they are "just chickens" and I'm reading too much into it?!!
I don't know. Again, I'm a newbie with a lot to learn but this is what I'm seeing with mine and I wanted to pass my opinion along. Good luck!