You should be able to go through the US line together, however in reality, it will depend on whoever is working the immigration line that day. We've gone through together at Newark quite a few times, but were told that my husband had to go to the non-citizens line when we got to the front of the US line at Logan. I don't think this was correct, as I saw a US/UK couple a few people ahead of go through with no problems (they just never got questioned while in line) but in the end, we didn't really want to argue with this guy. He did say that I could go through the other line with my husband, but I just went ahead so that I could get the bags.
We generally use IRIS when we enter the UK, so no separate queues.