I give to several friends who have a significant other, but I give only to the person who's my friend, and don't even think about the partner. Likewise, I receive a gift from, say, my friend Tina, with no mention of her husband Tim being involved in it.
I feel like it's not so much a rule
as the pattern set by mutual, if perhaps unstated, agreement (a sticky subject sometimes, to be sure). Kind of like if I get Tina a large, pricy piece of home decor, and she gets me a set of mini office novelties (clearly smaller and less expensive), I figure she wants to reduce the amount we spend on each other and I step my gift down the next year. She's never given me anything "from Tina and Tim," so I feel no obligation to find a gift for Tim as well. Besides which I know Tim far less well than I do Tina.
One interesting scenario involves my friend Amy. I've actually been friends with her husband Adam for longer
than I've known Amy and at one point, years ago, Adam and I were pretty good
friends. But, we were not gift-giving friends--he's not really a gifty person, if you know what I mean. Now Amy and I are much better friends than Adam and I. Amy gives me a gift from just her. I give a gift just for Amy, and I also give a gift to their young child. So I'm actually giving gifts to two-thirds of the household, but not the final third. Even though I've known Adam the longest.
But I think this is what everyone feels comfortable with, and to me that's more important than trying to enforce abstract "rules."