Hi Meg!
Congrats, those kittens looks so adorable (great markings too)! :)
As for getting them socialized with each other... Even if they're from different litters, wouldn't the breeder let them socialize with each other in his/her cattery? When I got my baby girl a few days ago from my breeder, I saw lots of little kittens walking around in the house. I understand that since they're both really small, they're probably kept in different rooms right now, but my breeder said that he lets them roam the cattery once they're not too reliant on their mothers anymore. But even if they've never socialized inside the cattery, I think they'll be fine if you just put them together, since they're both really young and entering a new environment, so territorial issues won't really exist.
Regarding you and your kittens- When you get them, definitely spend lots and lots of time with them. I feel like the more time you spend with them, the quicker they warm up. Make sure you place them in their litterbox in their safe room and allow them to make their way out of it themselves so they know where it is, and make sure you use the same litter as the breeder. And also, the first day or so of bringing the kittens home, feel free to leave out some food (again, same food her breeder was feeding her), but don't really expect them to eat anything. Mika didn't eat anything until I offered her some wet food the second day, and she didn't really start playing with toys until the third day.
Hahaha, I feel like I'm starting to rant now... I'm pretty new to this too, so know how exciting it is to be getting kittens. :) Don't worry too much, and feel free to ask lots of questions on this forum. Looking forward to more pictures of your kittens when you get them!
-Sara