I have some interim reports. It's about 3 months in to the burrows and the "second seven" have no friends yet. In that time, I got one "lover" relationship in the first seven. I only have 4 other immigrants so far, though. I'm 9 months into the fortress total.
Each burrow has a meeting place, dormatory, and tavern. I've pretty much segregated the work, but at the moment the work places are in the same general area. This means that the first seven and the second seven will bump into each other in the corridors. I also have many shared stockpiles. I'm just in the process of setting up temples. I've decided to go with individual temples for each god. The temples are segregated, so because some gods are worshipped by both areas, I will have multiple temples for those gods.
Based on some reports, I was kind of expecting some crossover in which taverns people attended, but so far they are obeying the burrows. The taverns (and temples) are in out of the way places, so neither side has to cross over a burrow to get to a tavern (which could distract them, I guess). I've also disallowed any visitors at all. It's nice to see them reciting poetry native to the civ for once!
Finally, I was also careful to construct the taverns and temples from dining halls rather than meeting places. I have exactly 1 meeting place for each burrow. This seems to solve the problem that FantasicDorf reported that dwarfs never go to meeting places. Indeed, I have dwarfs idling in the meeting place rather than the tavern. Now they only go to the tavern to socialise.