That's an interesting question. Most countries tax husbands and wives independently. In addition to the example of the French system, there's the U.S. system, where most husbands and wives are taxed jointly. For example, when teaching I find that a lot of people are aware that's how it's done in the U.S., and the question arises of whether we should we think of taxing the family unit rather than the individual.
Actually, I had an experience with this back in 1992. I presented a paper at a conference arguing that we should think of doing this, or at least figure out how much inequity we're introducing by taxing people separately rather than as families. There was a very strong negative reaction from people at the conference. There was a very strong feeling, not just from people at the conference, but also from others I spoke to back in the early nineties, that the independence of husbands and wives was too important and too fundamental or central to our outlook at the moment to think of aggregating their income and taxing them as a unit. So I've been discouraged about such initiatives.