We were in Boston for Thanksgiving. I'm American, husband British.--we fill out one US customs form (the rule says one per family). I have taken to asking an offical looking person which line to go to each time we travel to the US. Rules seem to differ depending on homeland security and the particular airport... This time around, they told us both to go to the non-US line. Last trip (O'hare), they sent us to the US line. Go figure. My advice: just ask politely before you get in line!
In the UK, my husband always goes through the Non EU line with me (He's nice that way). This time around, the resident/visa holder line was open! Imagine that!
Hope this helps,